I think Suzaku may have had some suspicions about Lelouch being Zero for a while but never really put much stock into them until about the phone conversation at the end of 23-- I can't figure out any way that Suzaku's "You're doing it for Nunnaly?" line makes any sense if he doesn't think that Lelouch is Zero at that point. I kind of doubt his suspicions were anything serious until that point or perhaps a bit earier given that the fact that Euphemia was so willing to work with Zero was another big hint; either way I don't think Suzaku really suspected beyond something like "hmm, that kind of reminds me of Lelouch, but that's not possible" until at least that point. Anyhow, even then he hadn't confronted Lelouch directly and hadn't completely confirmed his suspicions (he still doesn't want to believe it), so when he's face-to-face with Lelouch/Zero there's no way he can just ignore it anymore.
In large part it's probably partly a "hindsight is 20/20" kind of thing, there were definately hints and signs as to Zero's identity since very early on, but I really don't think Suzaku genuinely suspected him until much later, but having picked up on some of the hints earlier and immediately discarding them gives the sense that he did know for a long time (from Suzaku's perspective that is, kind of like if you notice some detail on a mystery show implicating character A but then immediatly dismissing it for one reason or another and then at the end when it is revealed that character A did it, even though you may not have been really suspecting them prior to that point having picked up on that early hint makes you feel like you figured it out earlier).
And that was much longer than I had intended it to be....
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In large part it's probably partly a "hindsight is 20/20" kind of thing, there were definately hints and signs as to Zero's identity since very early on, but I really don't think Suzaku genuinely suspected him until much later, but having picked up on some of the hints earlier and immediately discarding them gives the sense that he did know for a long time (from Suzaku's perspective that is, kind of like if you notice some detail on a mystery show implicating character A but then immediatly dismissing it for one reason or another and then at the end when it is revealed that character A did it, even though you may not have been really suspecting them prior to that point having picked up on that early hint makes you feel like you figured it out earlier).
And that was much longer than I had intended it to be....