Their house was super decorated, too. But we'd go over to my brother's and everything would be down on the 26th. It was depressing.
Growing up, my mom always insisted it was bad luck to have the Christmas tree up on New Year's, so we would take it down NYE. She gave up on that eventually (probably around the time we got a fake tree) and she keeps it up longer.
I've kept mine up until February before, LOL. The outside stuff at our condo had to be down by Jan. 15th but the last few years we rarely decorated, either outside or inside (just no room). I think we'll probably take down most everything this weekend.