I think that's plenty of time for the first few books, especially since a lot of the books rely on narration which I assume the show won't have as much of. I know people hate the movie, but it did manage to fit in a good chunk of the first three books in two hours, so it should be easy to fit one book in one hour.
Apparently the audiobook for the first books is only two and a half hours. Again, assuming they don't have as much narration on the show, plus that they'd obviously at least cut out a lot of the "he said" and descriptions of people doing things, seems like one hour would be enough. By the fourth book they might bump it up to three episodes.