The longest I've done is 10 days at Fort Wilderness after my senior year of HS when I latched onto the family of my then GF. It was actually quite great. Like has been mentioned before, the longer trip really allowed for more adventuring. We took about 5 hours one evening and took the monorail tour and stopped at each hotel and explored. We visited every park at least twice with the park hopper. We visited both water parks, DisneyQuest, both mini golf locations. Spent a night at DTD. It really does allow you to see the things you can't convince yourself there is time for otherwise.
At the same time, I think it actually HELPED that we were camping. The more relaxed atmosphere and the fact that we had a bit of "home" with us in the camper really helped. But definitely at 10 days we were done and cashed out. I wouldn't go longer if I get the choice again, but do hope for a longer trip again with my own family.