Like others have said, you can do each park in 1 day.
If you go longer, your commando level can drop, and you can slow down.
It can be done either way. If you go commando style, you could to both parks in 1 day and see most attractions. Then again, people have stayed an entire week, taking their time, going to the pool, restaurants, shows, hanging out, and never get bored.
Visiting SWF is a good idea if you have extra time.
For WnW, be sure to arrive early, so you can get on the major slides before the crowds arrive.