Our two children are three years apart which is about the age span that we had decided on. And, while we talked about a 2nd child even during the first year after our son was born, I couldn't even seriously consider it or do more than just refer to "someday" until he was about a year and a half. I was just too overwhelmed with the first child, working full time, etc. At that point, we did get pretty serious about it due to the fact I was quickly approaching 40 and just didn't want to be pregnant any older than that.
I think if I had been younger we might have gone with a 4 year age difference. However, the three year age difference is really working well for us and I'm glad that's the way we decided to go.