Typically AK opens by 9AM, I would comfortably say they would let you in at 8:30 ish, or perhaps even before. I remember people showing up at MK an hour before their reservation for breakfast and being let in. This of course was before they changed the pre-show/holding areas.
I've heard two accounts of folks with reservations up to 15 minutes after opening who were let in before the park opened, but only at Magic Kingdom. The long-standing policy is, if your reservation is after opening, then you don't get in early. The parks usually open a few minutes early anyway.