Remember that there are 6 parks when you add in the water parks.
However, one of the most commonly forgotten things about DIsney is that there is so much more than just the parks.
If you don't have kids, there are behind the scenes tours. They happen in the parks, but some take most of the day.
There is water boating, parasailiing, fishing, regular sailing, jetskies, surfing lessons, etc.
beaches, pools
Diowntown Disney
The Richard Petty Experience
World of sports
Miniature golf
regular golf (PGA tour courses)
Spa's
Outdoor movies
campfires
arts and crafts, storytime, etc at many of the hotels
and that's just scratching the surface.
You could do something different every day for a month and not run out of things to do. Easily. Probably longer.
The real question is: how many, what age and what interests are in the family?