There really is no *right* time. My older two are 11 months apart (no planning..more like surprise) then 2 of them (not bio) are almost the same age, then there is a huge gap between the older and the younger, they are (the younger ones) 2-3 years apart. Each is different, when we had #5 we thought we were done, but then we realized that the older kids had each other growing up so he needed a playmate, so along came #6, then the last was a sorda oops, but my husband still wanted one more so for him no biggy.
I always say you know when it is the right time, just don't over think it otherwise it may never happen.