If it were me, someone who doesn't like to cut things close, I would probably get to the bus no later than 7:30. That gives you a cushion in case a bus doesn't come for 30 minutes or so and also gives you time after you arrive at MK to get through security and the ticket gate and get up Main Street.
If you get there early and want to stay away from the first wave entering the park you could hang back for awhile until the crowd clears a little. But, at least you will be at the park.
If you're the type that likes to cut things close, you could get to the bus stop as late as 8:20 and still have a decent chance to get there in time, but, you might find yourself hustling down Main Street to get there by 9:20.