James, don't they have "mandatory" stops specified? I know that our 4 hour RDC Enduro, we specify that you need at least 2 ea 5 min pit stops, so that you can re-fuel the car safely.
Cars get penalized for fuel spillage in the pit lane, or if the crew members aren't wearing the appropriate safety devices.
So, the ideal setup is 3 drivers, driving around 75-80 minutes per stint.