Christmas decorations begin going up immediately after Halloween and are completely gone around January 10-12. They start playing Christmas music in the resorts right after Halloween, too. I don't know the exact date, but have heard it as early as November 4. And they keep playing the music through the first part of January, too.
IMO, the Christmas decorations at AKL this year weren't all that impressive - but I got sick as a dog with pneumonia while I was there, so that may have clouded my judgement!! I think there were less, though.
That said, what they do is distinctive - like the AKL itself. They aren't traditional American decorations. If you enjoy the AKL, you'll probably like them. If you don't, you probably won't.
IMO, the prettiest resorts at Christmastime are the Grand Floridian and the Wilderness Lodge. WL is my favorite, but that's a personal preference. It isn't that they do more - the Beach Club and Boardwalk do it up, too, but the color scheme of their hotels kind of clashes with the decorations.
Keep an eye on the Travel Channel. They have a show (or two?) about Disney at Christmas and show some of the resorts.