We were there for New Years a couple of years ago and in MGM on the 1st...We walked down the main street at park opening, and all the garland / lights were still strung back and forth between the buildings. When we came back through later in the day, they were already gone!!! I think they left the trees up though, just started taking some of the decorations down in the parks. Our hotel was still decorated when we left on the 3rd.
It was actually pretty interesting because on that trip, I did the Dophins In Depth tour at Epcot and I asked our tour guide about the decorations. She said they get nearly everything put up in ONE NIGHT. She said the giant Christmas trees were several pieces that screw together. She said that hundreds of staff work all night long to get the majority of the decorations up. It all sounded pretty neat and everything was beautiful.