If you want to be there "first" I'd leave your hotel no later than 7:25 if staying on property. You can probably be at MK by 7:45, then they let people onto Main Street at 8 but no further/not into any lands. You can go into the shops or get something to eat. If you just want to "rope drop" I'd leave my hotel at 8:00 and be at MK latest 8:30 so you don't have to wait too long.