I consider three days a . . . start. Easily, one full day for each park, to allow sufficient time to really take them in and experience everything, including one day for Volcano Bay (gotta remember to consider that one as a full fledged park). That leaves zero time to explore the resorts, Citywalk, make repeat visits to favorites, maybe a resort pool day, mini golf, and my favorite - water rides at the parks followed by pool time at a resort.
As others have pointed out, where are you staying? That makes a difference, on or off site. Lots more time to explore if staying on site.
Much like Disney there is a "tired" factor here - how much do you want to do in a day? If you like a casual schedule three days is not enough. If you are a commando, maybe three is enough, but you end up exhausted trying to squeeze in everything.
Personally, we stay at least five nights and even then we leave a lot undone. Someday I want to do a Schumi - 19 nights. Now we're talking . . .