I don't think there are any sure answers; just depends on what exactly they are (breed crosses) and environment/feed/exercise you provide. Seems like some folks here state that broilers won't last beyond the year-mark, but a lot of the comments I've seen are for the CornishX broilers.
I have 2 "red broiler" pullets that we are going to let grow a while, but they are too young (13.5 weeks) to know what they'll be like down the road. Maybe a few of us can test the waters and find out over the course of the next year? I'm willing to let mine stay around; they look nice and are quite gentle. We are free ranging them but they also get some feed from time to time, especially lately with the rough weather.