I think all kids are different, some will walk for miles, whereas others will tire very quickly and become grumpy, making life very awkward for the parents. A friend of mine has only really just stopped using hers and her son is 3.5 but she doesn't drive and has to walk everywhere so he would have hopped in and out as he needed to.
I stopped using the pushchair for my daughter around 2.5 as she would have walked everywhere. My son, well I don't know as he has had problems with his legs and isn't actually walking yet, so who knows, maybe this will mean he could want to be in his pushchair a bit longer than some.