Without spending a ton of time typing a response, we run 20-25 minute sessions for safety purposes (physical fatigue, mechanical car issues, etc.) for all run groups. In the six seasons we've used this format, most people like it.
One thing you'll notice, they are true 20-25 minutes sessions as we have a 5 minute break between sessions to clear the track for the next run group.
Hope you'll join us 2013+ at the track. Happy Motoring.