We've had the 8:15 and the 8:25 slot before...usually check in between 8 and 8:10am...seated fairly quickly. Beyond that, it depends on you.
We used to be able to finish up before 9 and see all of the characters, but on our last two visits, the timing of the rotations were way off and would have made it impossible to see all of the characters without sitting there for at least 75-90 min. We've done this one enough times that we just leave when we're done eating and don't NEED to see all of them, BUT...considering you're paying so much more b/c it's a character meal...it bugs me when the character rotations are run inefficiently.
Ultimately, if you don't care too much about meeting all of the characters, you should have no problem making it out by 9am. Just remember that everyone is being let in by about 8:45am when it's a 9am opening.