This question comes up a lot. It's not so much as to how long it takes to visit the parks, but rather how much time you want to stay (or can afford) at the resort. Some people can go commando mode and visit both parks in 1 day doing only the thrill rides. Others can spend an entire week hanging out at the pool, and relaxing without getting bored.
3 days is what most recommend, one at each park, and the last day for favorites before leaving.
And you don't have to visit only 1 park each day. You can visit one in the morning, and then one in the late afternoon. The only thing is, in September, the hours will be cut back, so the parks will close around 6pm. But you can still walk back and forth between them.