I too agree that you will just know....

I wanted another baby after DS, but it wasn't until he was about 18 months old that I got the physical urge to have another - and trust me it is physical, I swear my uterus would contract when I held a newborn PMSL

But while DS was little - eg from birth to 2yrs, I could not imagine having two little people to share my time with, personally I felt while DS was little I wanted to be there 100% for him. In the end our age gap is 3yrs3m, which has been quite smooth sailing. DS was old enough to understand why Mummy was busy with the baby, and old enough to amuse himself most of the time while I was feeding/settling etc. I don't think a younger child would understand or cope, but then I could be very wrong!