For a while it seemed like the trend was to have them three or more years apart, and now it seems to be more like 18 months, and I think the older mom thing probably has something to do with it. My first two are 18 months apart and the third is two years later, and I like them close. I got everything over at once. My brother's kids are six years apart, and I'd pull my hair out if I had to do preschool, jr. high and highschool twice. It's none of anyone's business how you space your kids.
As for your husband being done, well, mine was done after the first one, then after the second one, and then he actually came home one night and suggested the third one so I took him up on it really fast. He kept saying he was too old (he may have been right, at that.) But money issues are important, and teenagers are really expensive, so while you may want more kids, consider whether you're good at living frugally and limiting activities and vacations, etc., or whether you'll want your kids to be in every activity and to have tons of lessons and go to Disney every year. Neither answer is the right one for everyone, so you get to choose, with your husband, which is right for you.
Also, even when you're done, really done, you get that baby urge for years. I had to remind myself to stop and think, do I want another child, such as another 12 year old, or do I want another baby because I like to nurse and rock and cuddle. That part only lasts a tiny little time. What you have to think about is do you want to raise another person, not do you want to have another baby.