I was just looking at pictures of our trip last Christmas, and there are some decorations and trees still up at AK, MGM and The Wilderness Lodge on January 5th and 6th.
We went last year the week of the 7th and there was a lot still up. We came home the 14th or 15th can't remember which but the garland that went accross Main Street, USA was still up on that day. I believe some things started disapearing during the week though.
I went last year in early January and there were still some decorations up when I left (around the 14th or so, I think). There is no set date that they come down. They come down bit by bit.
It will start coming down after the new year. It takes a long time to take everything down. Each night they will take down a certain amount of things. Each hotel will take one night to take all the decorations down and the parks will each take a couple of days to do. (Just did the Yuletide Tour a few days ago )
..Went last year and was worried I was going to miss all the decorations, even though we booked from Dec 27 to Jan 6.
I was so glad to see everything still up and...so beautiful. Truely festive. I even got a chance to see Christmas around the world at epcot before I left. Simply magical and truely Fantastic.
Have fun! And if this is your first time going over the holidays..be ready for the crowds...their outragous.