You should be at the bus stop by 7 in case you should just miss one. Send a scout for the bus if you can.
We just missed one last week and saw multiple buses for other parks before another MK bus returned. We waited 30 minutes and again while riding while the driver explained to people at another stop that the bus was full and thus they would have to wait for the next bus (which was close behind us by that point). We were 15 minutes late. If the park doesn't open until 9 you should have better luck with consistent transportation.