The million dollar question!!!
My two are 3yrs1mth apart, not exactly by choice as it took time to concieve DD, but i enjoy this gap so much that i'm trying for #3 at the same gap. I have friends who have 18mth gaps, some with 11mths and 13mths, and one with 11yrs b4 her twins were born. And we all can describe the good and bad points of each of these!!
My DS is starting fulltime school next year, which means that my DD will get me all to her self for a while, then hopefully #3 will arrive.
I think that you will know when you are ready for another baby and that's the gap that will be best for you!
HTH