Sounds like my school is a bit of an anomaly. We drill\flow in the beginning of class, do technique, and then it's just free rolling. No one sets a timer. We just pick a partner and start rolling. We stop when someone taps but instead of grabbing someone new we'll just restart over and keep repeating the process for 5 or 6 rolls. However sometimes you can roll with an evenly matched opponent and can be rolling for up to 15 min without anyone tapping so then we might take a break for few min. Sometimes we keep going or sometimes it's the end of the night for one of us. I might change partners once if the guy I'm rolling with has to leave or needs to take a break. We have a no gi open mat on sunday and oddly enough we set the timer for 5 min rounds and just constantly rotate through partners. Even though staying with one person for so long might be unconventional it does allow you to really tinker with specific styles.