I think 6/20 min sessions makes sense for novices and low intermediates. Many don't have the tires, brakes, and physical/mental endurance to do 30 min sessions. Many don't have harnesses too so they get worn out a little more.
For an advanced student, 30 min is no problem. I would hope by the time you get to the advanced level you've learned to prep your car and yourself.
With that, I think it would be too confusing to run different session times for different groups. Which is probably why many DE orgs go with 20 minute sessions.
2013 F30 328i - Melbourne Red
2011 E90 M3 - Monte Carlo Blue
2004 E46 M3 - Imola red
Last edited by bigjae1976; 10-16-2012 at 01:16 PM.