I think the decorations stay up through the end of the first week in January. However, the wreath comes down from Sleeping Beauty Castle on New Year's Eve for the big show that night. I don't know when the lights came down from last year's Winter Castle.
The decorations start coming down the 2nd of January. We were there a couple of years ago and the tree was there one day and gone the next. Same goes for the decorations on the huge letters in the plaza. One day they were candy canes and the next day they were back to normal.