ETA: I think any time to have children is probably the worst possible time, except for all the others.
Ha!
Ours are 4 years apart. This was entirely intentional--we weren't even slightly ready to contemplate #2 till DD was 3, and even then we really only went for it because we didn't want a 5-year+ gap. In retrospect I think 3 years is likely a perfect gap if that works for pepole, but there are many reasons it might not. I'm happy with the gap we have for the most part, although it has made little-kidhood stretch on and on. DS just dropped his nap and this is the first time we have not had to worry about someone's nap since DD was born in '04.