We did it in UNDER one day last summer. (And a Sunday, at that.) We skipped a couple things, like the Waterworld stunt show, but saw most of what we wanted in a few hours.
FYI: We overslept and didn't get there until around 11, so we ended up buying special passes that give you access to the front of the line for certain attractions. It was GREAT! I walked onto Jurassic Park - twice - and we had NO wait at all for the tram tour. If you have the bucks, and don't have much time, it's DEFINITELY worth it.
"It's the time of your life, so live it well..."
Greg K.
Forest Hills, NY
Grand Floridian: '96,'97,'99
Disneyland Hotel: '00