We usually do 14 or 15 nights (depending on what time of day we can get flights) and find it perfect - if anything it is just enough time to do everything we want in a relaxed way. We are very non-commando, and like to do the parks in the mornings and evenings, with an afternoon break to go to the pool or enjoy the resort. We also enjoy adding some non park days to visit the outlets, other attractions, or just relax. We might do Legoland, Sea World or Universal for a day, and we also enjoy the water parks, Disney Quest, and recreational options (DS enjoys miniature golf and fishing at Fort Wilderness).
This year we are doing a 14 night vacation, but with a 3 night cruise on the Disney Dream in the middle.