Although I have seen posts to the contrary, I have found that the first time I try for a baby after a previous child, I get the "too soon" message. It doesn't matter if the youngest child is 2 or 12--if it is the first time they have tried since that one's birth, it seems to be "too soon".

So I have gotten in the habit of having them try right after mom finishes nursing the previous child to get that "too soon" thing out of the way. I am guessing the timer starts once you do that initial try after the previous child. As for how much longer before it is no longer too soon, well that depends on the desire for children, could be 15 mins, could be an hour or more.

Again, this is just the results of my experience with the game. I have seen posts by others that have different responses.