What is the proper age gap between kids?

Hi, You know, I never thought about what should be the appropriate age gap between kids till my daughter turned 6. As, now, I think that if one plans to have more than one kid, then have it soon, as in, when the first kid turns 2 or 3 year old. Because, what I feel is that if one takes longer time then you feel the whole process of delivering the baby and rearing him up too taxing. You do not feel like taking the plunge. And if you have kids back to back then you are mentally still dare to go through the whole process.