DS was potty trained just before he turned 3 and he was still wearing a 3. He had been in them for over a year.
I have a baby I watch right now who is 10 months old and the 3s are getting tight. He's ready to move up to a 4.
I don't think you'll get a consistant answer on this since it's going to depend on the size of the child, how fast they grow, etc.