Since we're DVC, we usually book our home resort at the 11 month window. If we want to switch to another resort, we go ahead and do that at 7 months.
However, for next year, we're doing an add-on of points in prep for a BIG stay in 2012. So we can't book next year's trip until we do our add-on. We'll probably be booking only 4 or 5 months in advance (which makes this über planner very nervous!)
But in general, our answer is 11 months!