You want to be in the park half an hour before they close the gates to the public (usually around 05:00pm).
I've visited IOA in the morning, gone to my hotel, took a nap, then came back, but I did it half an hour before the gates closed. If you don't get back into the park before they close the gates, you'll be stuck waiting. I'm sure you'll get in, but you'll have to wait with the crowds.
As you exit the park in the afternoon, get your hand stamped and hold on to your ticket. That will get you back into the park before they close the gates. Then you can walk to the stay and scream area and get a head start on the crowd.