I'd go for as long as you are able to with work/school schedules... the longer you are there, the more you'll get to explore.
First of all, the theme parks have so much more to explore than just the numbered items on the map. Stop to interact with characters, search for DeVine in AK, sit back and enjoy Off Kilter in Epcot's Canada Pavillion, grab a funnel cake to share with the family while you watch people or other things going on around you. There are so many performers that just pop up while you're just walking from point A to point B. Take some time to talk to the CMs. Chat with other families visiting the parks. Take in your surroundings... look for those little extras in the decor... search for hidden Mickeys... etc...
Also think of WDW as more than just the four major theme parks... there's water parks, DisneyQuest, the Boardwalk (fun at night), mini golf courses, boating excursions, golf, even simply browsing the resorts. Theming is simply incredible everywhere you go! Take time and enjoy it all! Book some dining reservations at different resorts and take some time to explore the resorts while you're there. Dining in itself can be an experience - that's why I try to experience new restaurants every time we go back. And by visiting different resorts, you can feel more a part of these resorts, even if you're not paying to stay there.
There is simply SO much to do in WDW, and not all of it has to be done in a commando fashion... don't try to cram all of one park into one day. You'll even find that if you do certain things a few times, you might enjoy them more at different times of the day.
Pace yourself and enjoy your vacation... if you're too rushed, you will likely return home thinking "I need a vacation from my vacation!" I've been there, done that... I'm a strong proponent of staying longer... especially if you can only visit once each year or less often.