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.

Cool video. Would I be correct in assuming the car that shows up a couple times but never passes is some sort of escort to keep them from being run over? If there isn't a posted minimum speed, is that legal? Also, is there a reason they seem to be rotating counter-clockwise? For some reason it seems groups usually go clockwise (i.e. pull on the left and dropping off to the right)? Wind?

Yes wind. You want the decelerating side getting the wind, thus protecting the faster side of the rotation.

Different group rides have different characteristics and sometimes they even vary depending upon who shows up from week to week.

Some group rides are hard, competitive race-simulation workouts. All bets are off there. Sometimes you'll have half the riders struggling and not taking any pulls while the other half is trading attacks at the front.

Some group rides are much more of a cooperative effort and have structured rotating pacelines.

Some group rides are more social. These may have a double paceline with riders pulling up to their comfort level and dropping back in pairs.

Random charity ride pacelines may just have a handful of strong riders pulling a very long train. Be careful grabbing the back of these. The accordian action can get sketchy.