As of yesterday, 8:30 am seems to be when the parking gates open. At about that time, there's maybe a line of ten cars or so. (According to the info the Team Member at the parking gate gave me.)
9:30 is when ticket scanning begins, for general admission and early park entry. That line looks to be about 20 minutes. Rope drop at 11 inside Celestial Park.
Early park entry (for hotel guests) gives you access to SNW and Stardust. Everyone gets access to Starbucks and I think a shop or two (unsure about this). Also not sure about how much of Celestial Park is open before 11am, and where to line up if you want to rope drop Monsters.