I can only give an estimate but I am sure someone who actually knows the dates will catch this thread and let us know. But I THINK the decorations will be down by either January 5 or January 12.
You know a good way to find out is to look on the DL site and find where they talk about Small World Holiday or Haunted Mansion Holiday and look at the end dates for those rides, sometimes it is in teeny-tiny print, and that means everything else will be coming down around then, as well.