Really does depend (on a number of factors not just individual strength). My preference is definitely a constant rotation so usually sub30 sec pulls. Everyone stays fresh and active like a swarm hellbent on the group goal. The reality is most groups are not evenly distributed in terms of strength/conditioning so something approximating a constant rotation develops with stronger riders taking a longer pull to help the group more. How big a difference is the "depends." The other big factor is knowing and trusting the other riders well enough to keep tight, even and consistent with the line and the traded pulls. Easy for teams but much more difficult for most of us schmoes who ride with ever changing groups...