Like it has been said, it depends on too many things: wind speed, terrain, length of the ride, speed of the ride overall, purpose of the ride overall, composition of the group riding, etc. Typically, I would say the faster and the bigger the group, the shorter the pull should be and vice versa. I also go by time and not distance as time is constant regardless of terrain or wind while the over isn't. I will admit it does irritate the hell out of me people that do sit on the back of a group and just let a small group at the front do all the pulling. It is one thing not to be strong enough to do your equal share of the pulling (I can relate to that most times), but it is another just opening a hole in the group for other riders to come in, which int arms makes a yo-yo effect on the group behind those pulling. If they just rotated to the front and came off after 20 sec (enough to allow the rider that just got off the front to move back far enough), it would still provide quite a bit of a break for those pulling allowing them to pull longer and even faster.