I think the question itself contains a pretty big assumption: That the answer is a binary. Either he believed her immediately and never seriously wavered in that belief, or he just plain didn't believe her, and essentially held onto that lack of faith all the way to the end of AtS.
In my opinion, Spike's perception of Buffy's "I love you" was complicated, and evolved both in the moment and following his resurrection, as he got distance from the moment.
In order to explain that in depth, I'm going to look where their relationship stood at the time of Chosen. To recap: After the horror so that was S6, Buffy and Spike made their peace with each other through the first half of S7, culminating in Buffy giving him her faith and Spike holding onto that through torture.
For the next several episodes, their relationship remained stable, defined by trust and affection. The relationship was platonic and low-key on the surface, but there was undoubtedly a particular emotional intensity in how they interacted with each other. Neither of them was prepared to act on it. For his part, Spike was overwhelmed by his new soul and fresh self-loathing. Combined with his history of trying to force himself on her in more ways than one, he didn't want to push with Buffy. As for Buffy, she knew firsthand - twice now - how dangerous it can be to get involved with a vampire.
Buffy had a lot of issues around her romantic history. She was concerned not only that she might be unable to love at all, but that she might, as she put it in First Date, be attracted only to the "wicked energy". Despite what she said to Spike in Never Leave Me about not being drawn to men who hurt her, she herself was worried that she was attracted to bad guys.
And her S6 relationship with Spike had been a complete mess that ended with his attempt to *rape* her. I don't think I can emphasize this enough. Yes, Spike's soulless state and subsequent souled state are a fantastic situation that makes me look at the rape very differently than I would, say, with Luke and Laura. And yes, Buffy's abusive behavior during the course of the relationship muddies things up even further. None of that changes the fact that Buffy in S7 is seriously considering re-entering a romantic relationship with a man who tried to rape her. Buffy's had it hammered into her all the reasons why a woman shouldn't return to her abuser - hell, she's done the hammering (see Beauty and the Beasts). With Angel she could draw a line in the sand: She loved Angel when he had a soul, when he was soulless he was a monster (although even then she had some issues, hence all the hammering of Debbie in BatB). Her perception of Spike's souled/soulless identity was much more complicated - it had to be. I know a lot of Spike fans dislike the use of attempted rape, and I get that. But on a Watsonian level, it happened, and many Buffy/Spike shippers simply do not put enough weight on the assault when judging Buffy's actions. I'm not just talking about the trauma of the experience, but the implications of starting a truly romantic relationship with a man who had done something like that to her. Even with the souled/soulless divide, plenty of fans had serious problems with it.
Yet despite these concerns, Buffy and Spike's relationship reached an entirely new level in Touched (although note that Buffy's joke about Spike having problems with the word "no" shows that she still had the attempted rape well in mind). Spike was being pushy and persistent for the first time in a long time - not on behalf of himself, but championing Buffy to Buffy. Buffy's speech to Spike in Never Leave Me that she believed in him and wanted him for more than the pain he caused her led to their stable relationship from Show Time onward. Spike's speech in Touched telling her he loves he for who she is and not because of her unattainability brings them to a night of intense emotional intimacy, intimacy on a level that they previously only came close to in After Life, but never quite touched.
So that's all well and good and terribly romantic. But what happens now? What happens after you've had an experience that intimate and intense with another person? You can't maintain that level of closeness all the time. As Spike wanted to know, what does it mean that it happened?
People give Buffy grief for asking if it has to mean something, but in that same conversation Spike unintentionally echoes Riley in his dismissal of her brave admission of her feelings: "Yeah...I hear you say it, but…" (how much do you wanna bet that sentence would have ended with "I just don't feel it"?) That was hurtful, too.
Neither of them intended to be hurtful. They were both, as Spike said, "terrified". Spike's terror is more straightforward and easier to read: he's afraid that it didn't mean for her what it meant for him. I do think Buffy's behavior contributed to his fears. It's not that she did anything wrong, but she did disappear without speaking to him, avoiding an immediate morning after. At the beginning of the End of Days scene, Buffy comes across as all business while Spike, with his "defensiveness and weird, mixed signals", is easier to read from an observer's perspective.
However, Spike's fears are only influenced by, and not dependent on, Buffy's actions. He tells Buffy, "It was the best night of my life." Spike's wildest dream, the dream he worked so hard for before giving it up as impossible, as something that he had, in soulless monstrousness, destroyed beyond hope of repair…that dream is coming true. Spike never just wanted sex with Buffy, but emotional intimacy. As far back as Into the Woods, he admitted to Riley, "Sometimes I envy you so much it chokes me. And sometimes I think I got the better deal. To be that close to her and not have her. To be all alone even when you're holding her. Feeling her, feeling her beneath you. Surrounding you. The scent ... No, you got the better deal." While he ended this quote by concluding that it would better to sleep with Buffy without an emotional connection than to not have her at all, he came very close to arguing that such a situation would not only be insufficient but worse than nothing at all. Spike had the sex with Buffy without emotional intimacy in S6. Now, in End of Days, Spike has had a night of both deep emotional and physical (though not sexual) connection. And this after the attempted rape, after his soul-informed worldview had told him this connection with Buffy was impossible. The idea that the previous night might not mean as much to Buffy was so painful that Spike clung to this lack of belief, just because letting himself believe and having his hopes dashed would be too painful.
But when Buffy admits to Spike what the night meant to her and he admits his feelings to her and she tells him he doesn’t have to be afraid, you can see in his eyes that he is starting to believe her.
And he asks, "What does that mean?"
Buffy's in the middle of a war. She just had a night of intense emotional intimacy with a vampire who spent the last century or so killing untold numbers of people, including two slayers like her, and, oh yeah, tried to rape her. She's under incredible amounts of stress, trying to hold together an army of young girls against a foe they don't yet know how to match. Her best friend has lost an eye. She's had a victory, thinks to her emotional connection with said vampire, but she still can't see the finish line. She's not remotely ready to decide what her night with Spike meant to her.
Note that despite her hesitance in assigning meaning to their night together, Buffy wants to keep the conversation going, to keep trying to figure something out. But Spike isn't ready. He's still too scared. So that discussion is put off for now. As Spike puts it, "Let's just leave it."
And before they can talk again, Spike arrives just in time to witness Buffy kissing Angel.
For some fans, this is the moment that Spike lost all hope that Buffy could ever love him. I don't believe that. The relationship between the two of them had already survived a horrific period that included her beating the shit out of him and him trying to rape her. (I wonder if Spike ever found out that Buffy was prepared to entrust him with Dawn not long after the attempted rape?) Despite this, they'd managed to reach a point where they shared a night of cuddling and deep emotional intimacy - something Buffy straightforwardly acknowledged to Spike. Was Spike hurt by the Buffy/Angel kiss? Of course. Did it cause him to lose faith in Buffy? Please. This relationship could survive anything.
As for Buffy, we see from her conversation with Angel that she's pretty conflicted about her relationship with Spike (though not, let's note, about Spike himself). She complains about Angel being jealous every time she gets a "boyfriend", and then denies that Spike is her boyfriend ("But…he is in my heart."). She tells Angel that she doesn't see herself having "fat grandchildren" with Spike.
This is another reason for Buffy to have mixed feelings about the relationship. Sixteen-year-old Buffy may have insisted that she didn't care about children, that all she wanted was Angel, but grown-up Buffy looks back on her relationship with Angel more cynically ("What was the highlight of our relationship? When you broke up with me or when I killed you?"). The possibility that Buffy might want biological children (or adopted children - I can't see Angel or Spike passing a home study) isn't something that comes up much in shipping meta (I don't know if shippy fic addresses the issue more, as I don't read much fic). Because of Wood, Buffy now knows that at least one slayer had a child. Buffy at twenty-two is one of the longest-lived slayers of all time, having survived death twice. Whether or not to have children, when, and how many are among the most significant decisions any couple will have to agree on. By being with Spike (or Angel) Buffy would likely be throwing that possibility away. I'm pretty sure she knows nothing about the Shanshu prophecy. This is just one more complication in her relationship with Spike that makes her uncertain where she wants it to go.
As to why Buffy kissed Angel, I'll quote elisi: "See when she dated Angel everything was simple: They loved each other, and it was nice. Which is why I think she falls into Angel's arms - he’s uncomplicated and she knows where she stands with him. She can give him a kiss, but it doesn’t change anything. It’s a bit of a dream come true, and she wants to indulge herself, just for a minute. Or 5 seconds, rather."
I myself have a far more cynical take on Buffy/Angel. Buffy/Angel was a fantasy. Buffy/Angel was always a fantasy. They truly loved each other, yes - although honestly, re-watching season two Buffy and Angel do not read as in love to me prior to Surprise (I wonder how they would look back on their relationship if things hadn't promptly gone to hell). But despite his feelings for her. Angel kept huge amounts of information about himself from Buffy while they were together. She found out about his vampirism by accident. He was cryptic and misleading about Spike in School Hard even though his storehouse of information about William the Bloody could have made the difference between life and death for Buffy. He only told her his history with Drusilla after she caught him in a lie and even then asked her if she loved him before telling her anything. He outright lied to her when he says that he never fed on a living human while souled.
Their relationship was based on a fantasy: the hormone-driven high school sweethearts who were in fact a two-century-plus-old murderer and an adolescent girl destined to live violently and die young. Both Buffy and Angel found the fantasy appealing. Buffy made Angel feel like he could be good, that he could let go of his past. Angel made Buffy feel protected and cared for when the rest of her life was about danger and burdens of responsibility. But as Innocence cruelly revealed, it wasn't real. The massive power imbalance was that Angel knew it was a fantasy. He indulged in it, but he knew. Buffy did not. She was a kid. She didn't know any better. And Angel tried to maintain her ignorance.
Adult Buffy is not so naïve. When she indulged in the fantasy in Forever and in End of Days/Chosen, she knows exactly what it is: A comforting story. It's a story that in Forever helped her with her grief ("I didn't think I was gonna be able to make it through the night," she admitted to Angel), but a fantasy nonetheless. And in End of Days/Chosen, I'm not even sure she found the fantasy that comforting. It's Spike who stays with her through the night and gives her strength this time.
But of course, there is Buffy's the admission at the end of their conversation in Chosen that she does think into the future, hinting that she sometimes thinks of ending up with Angel. On a Doylist level, it's throwing a bone to the shippers. On a Watsonian level, it's Buffy not quite being ready to let go of the fantasy. And maybe she never will. Maybe there will always be a little part of her that likes to imagine Angel is her destined true love. But I don't think they would ever really get together, because frankly, I'm not even sure they really want to. They're in different worlds now, and I suspect the fantasy itself is just too reassuring - a real long term relationship would put an end to the fantasy. Even when they were together, the day-to-day reality of their relationship wasn't always that great. What follows is my highly biased opinion, but: in most of S1-S3, Angel and Buffy for all their intense attachment seemed to make each other miserable more often than they made each other happy. With the notable exception of when they were making out. No wonder Angel experienced his moment of perfect happiness when they finally had sex, and no wonder their last two on-screen reunions, Forever and End of Days/Chosen, included kissing.
I don't know if I want Spike to be "long-haul guy", so to speak, but Buffy's relationship with him is both durable and real. Weirdly, Buffy and Angel seem more comfortable with each other as exes. Which is one reason why I would never, ever, ever want them to get back together.
Back to Chosen. The day ends the same as the previous one: With Buffy and Spike together. Spike's pissed about the Angel kiss, but for that night he and Buffy choose each other.
Their relationship is still tentative. They're only beginning to figure out how things will work, now that they've had that intense experience in Touched. A relationship deepening in the middle of a war, despite or because of a complicated and painful past. It's still very tentative.
So of course, Spike dies.
Buffy would have said it, eventually. But it wouldn't have been then. It would have been after their shaky new intimacy was more stable. It would have been when she was ready. When Spike was ready.
But life doesn't care if we get the time we need. And so Buffy clasps Spike's hand, and gives him want he wants most in the world.
And that's the problem, right there.
Spike knows that Buffy knows that this is what he wants most in the world. And maybe Buffy's saying this not because it's true, but because she's trying to give him something, some reward. Like that kiss in Intervention. It's insane of Spike to think that, of course, because Buffy clearly means every word. But remember what I said about Spike going with the hopeless option because he can't handle the possibility of getting what he wants most?
I don't think doubt is Spike's first thought. Looking at his face, I think his first thought is just amazement. Amazement that she said it, that he is hearing what he is hearing and seeing what he is seeing. And only then do I think it crosses his mind that maybe, maybe she didn't mean it. He says without thinking, trying to maintain his pride, "No, you don't." And then he realizes he's kind of being an asshole, rejecting what could be a heartfelt and difficult declaration. So he tries to soften his words: "But thanks for saying it."
Buffy has said what she needed to say, and they have no time for an argument. Spike tells her to leave, and that's the end.
Except it's not - or at least it shouldn't have been. Spike comes back to life, but in a whole season of AtS he doesn't even let Buffy know he's alive. Don't get me wrong, I like AtS S5. I like Spike's presence. I like Damage in particular a lot. But I just can't believe that he would let Buffy believe he was dead. Sure, I could buy them agreeing to go their separate ways, at least for a while. But allowing her to believe he was dead? Even if he really didn't believe that she loved him, he knew she cared. He knew thinking he was dead would hurt her. And he loved her.
(I think he probably did believe she loved him, most of the time. I think that moment of insecurity was just that, one poorly timed moment. I think with distance he realized that she wouldn't throw around a word like "love" casually, and that the look on her face and her tone of voice were proof that she meant it.)
So I can't buy that he wouldn't tell her he had survived. I've heard that the showrunners were holding off in hopes of getting Sarah Michelle Gellar, but still. Even an off-camera phone call would have been better than nothing.
In short: I think Buffy meant it completely, and I think in the moment Spike was scared to believe her. She wasn't ready to say it, but even more, he wasn't ready to believe that she loved him. But I think in the long run, in his heart of hearts, he knew she did.