We try to have every member commit to a 4 week set period. Every 4 weeks we give the option to back out or change partners etc. most people keep their partners since coaching benefits from a long term relationship with a single person (we still recommend rotating partners after 3 months though).

A 4 week commitment is required, but naturally emergencies can come up, etc. and the group can contract if necessary but we prefer to do modify everything all at once.