I had 2.5 years between the first two, then 4.5 years, then 7.....
If I'd had my way with DD2, I would have had her sooner, I thought 2.5 was pretty good - but I looove the gap we have now! for so many reasons.
I think mrsmac is right, we adjust to what we have. So I guess just start trying when you feel ready to do it all again