How long after your first child/children did you wait to have another baby?

My husband and I have twins who are now three. My husband said he could settle with being done having kids because we have a boy and a girl, but he said either way he wouldn't mind one more. I think now would be the perfect time to have another baby, our twins are both potty trained and growing more independent each day, however they are not old enough that there will be a big age gap between them and a new sibling. My husband says he wants to wait until they have been in school a year or two to have another child because they are still so young and require quit a bit of time and attention (which is no problem, we will always lovingly give them that). I disagree with this, I would like them to be closer in age. What are your opinions on this topic?