The Grand Floridian tops my list at Christmas. I've also enjoyed The Dolphin, Wilderness Lodge, Boardwalk, Yacht and Beach Clubs.
As to time, that would depend on how you plan to get around, where you are staying and exactly when you will be there.
We were there early this past December and stayed at the Swan. We walked to the Dolphin, Boardwalk, and the Yacht & Beach clubs. We saw all of those one evening after returning from the parks. We spent somewhere around 1 to 1-1/2 hours there, but that included stops on the Boardwalk itself.
We drove to the Poly to view their decorations and rode the monorail around to the Grand Floridian and Contemporary. That was somewhere around an 1 to 1-1/2 again. We drove to the Wilderness Lodge and the Animal Kingdom Lodge after that. We viewed the lobbies and grounds at both of these resorts, very impressive but the WL had much better Christmas decorations, IMO.
We also visited the Caribbean Beach and Pop Century, but I don't recall being impressed by any decorations at either of them.
I'd suggest breaking your visits up by resort region and visiting the resorts while you are in the area. If you are going in value season, the parks will, generally, be closing early and a visit to the resorts is a nice way to spend an evening! You might even consider a meal while you're there!
Have fun!