It really depends on how much and how quickly your kids eat. I had an 8:05am with 2 adults and we were stuffed and saw all the characters by 8:50. They even took extra time when we checked in to decorate table for my brothers birthday and we still made it in time.
It also depends on the characters. CP is a really big restaurant, so it can take time to see all 4. You might be out by 8:50, but you might also need to wait until 9:15 to see Tigger depending on where you sit and rotation.
It really depends...we've had visits where we've seen all of the characters at least once within the course of an hour, but other experiences where we were pushing the hour and a half mark and still waiting on one or two.