They are up I believe through the 12 days of Christmas, so I think they take them down somewhere around January 5th or 6th.
Never having gone in December, only January, I'm unable to compare. I'm not sure if they are ALL up all over the place after Christmas. We did see quite a bit of decoration in the resorts, but I don't remember seeing much in the parks themselves. We also stayed at BWV.
The decorations will be there however keep in mind that other holiday related activities will be gone. Things like the CP at Epcot, carolers, Epcots holidays around the world shows, Illuminations holiday ending, tree lightings and a few other things will not be performed while you are there.
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