For me and TTC #2 was pretty much once #1 was born. My 2 DD's are less then 11 months apart!

As LG said...you just seem to know when the time is right and feel comfortable. Weigh up the pros and cons....think of future plans, can you afford to have kids close in age, will having them close be hard to look after, time with each child etc.

The gap between #2 and #3 for me is around 14/15 months...so again....nice and close