Most of the decorations will be up by the week of Thanksgiving. We went the week PRIOR to Thanksgiving and saw tons of them. The Mickey's Very Magical Christmas Party is starting on Nov 12th this year, so all of the Magic Kingdom will be decorated. The Gingerbread house at GF was up by Nov 12 last year, the Osbourne lights will also be starting on Nov 12 this year as well, so MGM will be decorated! Epcot had the Christmas tree up but some of the lights around the world weren't lit yet! But all in all if you go just prior to Thanksgiving you will definitely see Christmas decorations!
Hope this helps!