Generally, I pull for as long as I can sustain the avg speed of the ride, plus/minus 2-3km/h. Some of the stronger guys pull for 3-5 mins, others pull for 30 seconds. No one in the group seems to care as long as the momentum of the group is sustained.

If a guy pulls for too long, bonks and slows us down, we generally let him do so until he pulls off. If he gets dropped as we speed back up, too bad, should have pulled off sooner.....that's our group policy.