At our school, airborne time is measured by a Hobbs meter activated by an air switch. This time is used in the engine, airframe and prop logs for maintenance purposes. We then add 0.2 (12 minutes) to this number for charging purposes, so a take off to landing flight of 30 minutes would be charged at 0.7. This is a good arrangement since it allows people to taxi slowly and do a proper warm up without having to worry about extra cost.
Pilots log the actual time from chock to chock, usually rounded to the nearest 5 or 6 minutes. This frequently works out as a longer time than the time charged for, especially for students who take a long time doing the power checks and who quite correctly taxy very slowly over the rough grass areas at Redhill.