I've had the 8:05 am reservation on three separate occasions and we were always done by around 9 am on the dot (including a restroom break immediately following the meal). For each of those meals, I seem to remember the last princess visiting our table around 8:45ish.
A key I've found is to arrive early enough so that you're one of the first people in line (i.e., get to the podium well before 8 am). In that regard, the line to get in really builds quickly (due to the photo opp bottleneck with Cinderella prior to going up to eat).
On top of that, the princesses who visit the tables tend to come out in shifts. So if you don't get them all in the same shift, it tends to take a bit linger (another reason to arrive early enough such that you're not waiting between shifts to see all of the princesses).
Another thing to note is that the wait staff here is typically very motivated to turn tables. So slow service is probably not going to be an issue.