I've been enough times that 7 days would be too long, but I live within a reasonable (2&1/2 -3 hrs) driving distance, so I can leave here at 8AM, check into my hotel, and be sitting in a park having lunch by noon.
Most visitors have to deal with long distance travel, and many have jet lag. That tends to slow you down. I'm also used to the heat, humidity, and sun so am not as wiped out after a few hours in the park - but I do take time out of the sun either in my room or in the shade by the pool.
Trust me, a nice shower and a change into fresh, dry, clothes can really recharge the batteries. Just being away from the crowds and noise for an hour or two is rejuvenating.
So for a first time or infrequent visitor, 7 days would not be too much time, especially if you have any intention of seeing anything beyond the 4 parks, such as a water park, Downtown Disney, Boardwalk, miniature golf, etc.
You might want to spring for LaNouba, or maybe a dinner show at one of the resorts. It's also nice to have to to revisit a favorite attraction, or go back to see something that was closed (for a glitch) or too crowded the first time you tried.