Last time I NIP was when my DD was 2. I'm going to say that sometime around 18 mo or so (maybe even before) she got very distractable and didn't really ask to NIP too much - she was very occupied with everything else going on. And if she did ask, it was at a time where I was busy and would have told her to wait even if we were at home.
ETA: Probably around that time is when I stopped doing "on demand" as well. Not that I was trying to wean or not nurse but if I'm in the middle of doing laundry or the dishes or cleaning the bathroom, you can wait 5-10 minutes kindof thing. I didn't really start trying to wean until she hit 3.