Onsite guests get Unlimited Express as their perk, so for the most part, there is no early entry. So, when they open the gates, everybody is let in at the same time.
But, my experience is that they usually open the gates about 10-15 minutes before the scheduled opening time. What is even better is that usually the atractions in Marvel are already open and running (and possibly Seuss, not sure.) I remember one time, when I rode all 3 Marvel attractions before 9am, and they were blocking off Toon Lagoon until 9am came.
But definitely get to the gates 20-30 minutes before opening and hit the Marvel attractions first thing (Hulk first, then the order doesn't really matter, but I'd probably hit Spidey and then Doom), it'll give you a nice head start on the day.