Decor will be taken down all at once and in stages at the same time. They don't have enough crew to clear out all the Christmas decorations at WDW in one night, but they will clear out an entire area overnight. So you may go to bed at AKL with the tree and decorations all up, and wake up to find no traces of Christmas left at AKL. But AK itself may still be fully decorated. I've done Marathon weekend a bunch of times and while it varies year to year, most stuff will be gone by like the 10th of January. Background Music and landscaping may be slightly off, so the garlands/tree may be gone, but poinsettias still in the ground and Christmas carols are still playing, but they will fix that within a day or two.