Are these with the same couples or with different couples? How soon they will want to try again depends upon each parent's desire to have children. If you have parent's that are at the "not really" or "no way" end of the scale, you'll probably have to wait a lot longer between babies.
Oh! That's why some of my couples are willing to try for a baby when the youngest is only 3 years old, but other couples refuse no matter how many times I pick them up and put them together. I normally pick peeps that are only Maybe or Would be Nice and then try to combine them with someone that is No Way or a Maybe. I don't like lots and lots of kids, lol! But I do prefer 2 kids over just 1 -- gives you more to choose from when the time comes.