First of all, I presume you are talking about December
'06, right? Early December is possibly the busiest time of the year at
DVC and if you don't have ressies yet for this year, you have a real uphill climb.
Even in '06, depending on what accommodations you want, you may have difficulty getting what you want at seven months...so you may be limited to your home resort(s). It's probably a good idea to book a backup at your home resort at 11 months, in case you can't get what you want later.
As far as how long, we originally made ressies this year for three nights and then added a night. There is so much to see and do we did not think we would have enough time with just three nights. Even with four nights, we'll probably skip Disney-MGM and the Osborne Lights. We'll be moving a little slowly because we have MIL and her sister, both of whom are around 80. (They'll be slowed down by having to wait for me!)
But we're doing
MVMCP, CP, and some good diining, plus seeing a lot of the holiday decorations, and of course doing MK and Epcot. We can do Osborne Lights another time, and AK is possible, but not likely this trip.