As we've already seen, he'll be popping up in the second as Aleksei Sytsevich, aka The Rhino, and even earning himself that nickname by getting in some kind of powersuit. I assume that's how he gets the name, anyway, and not the scene where Peter Parker pulls down his trousers…
According to Giamatti's comments to Collider, he's already set to return in the third instalment. Beyond that? Well, while he's got a multi-film deal that covers a number of eventualities, Giamatti doesn't yet know if he'll be in the fourth film.
The Sinister Six are definitely coming to the series, I'm one hundred percent clear on that, and it's actually just a question of when. I'm not sure Giamatti's comments really answer that for us, but one could speculate that he knows he'll be in the third film because he knows that's where the Six will appear.
Yeah, that's just a guess. As good as useless.
Anyhow, it's nice to know he's sticking around. His scene at the start of The Amazing Spider-Man 2 is apparently pretty brief and relatively light. If it leaves us wanting more we'll at least know that the next picture will deliver.