With DS#1 it was around 15 mos, and then with DD#1 I didn't get it until DS#1 weaned, and I got it the next month after he weaned.
So I guess DD#1 was about 15 months, too, since DS#1 weaned at 3.
Then with DS#2 it was the same thing, I got AF the next month after DD#1 weaned, and she weaned at 3.5, so DS#2 was somewhere around 18mo.
Then with DD#2 it was the same thing, got AF the month or so after DS#2 weaned, and he was 3.5, too, so DD#2 was somewhere around 16mo. Or so.
I think it's not necessarily demand. It's more like the amount of time that goes between nursing. If your kids all have several nights of deep sleep, for example, where you go at least six hours at a stretch without nursing anyone, then the hormones for AF/fertility are likely to start kicking in. Or something like that. That's the rule that I've read, that six-plus hours with no one nursing gives the body the 'signal' to reload, so to speak.