there's really no rule. Things just fall into place on their own. The ride I like is on Staurday mornings and those bastards, every single one of them is out for blood pushing the pace all the time. Their goal is to keep it between 25-30 mph for the entire 55 mile ride. I am getting too old for that siht, and not enough time and miles to stir up the pace up front, so I suck wheels most of the time but will do my turn when it comes up. Usually I try and stay up near the front (top ten places) to avoid the less experience riders so they don't cause me to break my collar bone.
I really like the smooth rotation where the guys go up and do their turn of 15-20 pedal strokes. Some may do it for a bit longer, but at that pace, everyone's trying to save themselves for the return trip home in the hilly sections. The trick is to not burn yourself out and get dropped. No fun. I would say, 30 seconds to 60 seconds is acceptable for me. most guys, myself included, don't care if you go up and take a pull or not. They are out for a good workout or fun fast ride. There's always someone willing to do the donkey work up front.