Most grass won't grow too high even after years. But yes, that does grow quickly. I was referring more to the bushes and stuff covering the first story of a building that was mentioned.
I wasn't meaning the barricading takes a long time. That can be done in a couple hours unless you really get elaborate (some POI are pretty elaborate). I meant the people who were barricaded being killed or turned taking time. Decent barricades and weapons should hold off the initial attacks and just looking at most POI, most don't look like they were overrun by zombies, suggesting that most of those people probably died going out to hunt or get supplies, which isn't likely to happen to everyone in a short amount of time. At least until zombie numbers start to get really high.
But yes, there isn't anything saying how long it has been and one guess is as good as another. I just can't see it being under a few years or we would see people besides the traders. Unless the idea is that only this area of the world is like this and other areas do have a lot of people still (we get air drops, so this is a possible scenario).
Either way, I think limited overgrowth in certain POI would work better than trying to overrun the entire map.