Being a young adult (college student,) I'm not able to stay for very long on my *own* wallet, however fortunately my folks (and my boyfriend's folks) haven't cut me off from "family trips" yet, haha, so when we go with family we usually stay for about 6-7 days, on property. When left to our own devices we can afford between 2-4 nights usually (since we also have to pay for airfare, which from where we live is about $200 per person, give or take $50 depending on time of year and special and stuff...)
I've found that either way works, really - both my boyfriend and I have been to WDW a LOT, so when we go for a couple of days it's not so bad if we have to cut a couple of attractions out or skip one of the parks - we're mostly there for the atmosphere and to just soak it all up and enjoy not being at work or school (well, and to see PotC and Splash, our two fave rides, hehe!) And we also know that we'll be back again, so whatever we miss we pick up on the next time. But it's also really nice to get there and know that you HAVE 6 or 7 days to get stuff in rather than having that feeling of "Oh my gosh, I just got here and I have to leave the day after tomorrow!"
OK, enough of my rambling...for now...LOL