In my family it was a tradition to leave it up until the Epiphony (little Christmas) which I think is either Jan 7th or 9th. Normally my parents would put it up the weekend before Christmas. It was normal to frantically be decorating the tree and the rest of the house until midnight mass Christmas eve....but somehow Mom always made everything come together.
This year our tree went up the weekend after Thanksgiving and is getting VERY dry. I don't think it's going to make it to the first week of January.
I remember Mom telling me a story of a tree they left up for 4 months when she was a little girl. They were leaving it up for her cousin that was serving in WW2. He wanted to see a Christmas tree when he got home. He didn't get home until the middle of March. There wasn't a neddle left on the tree, but all the ornaments and lights were there (yeah, fire hazard!!), but he cried like a baby when he saw it. Just wanted to share that.