Members are eligible for reappointment, but shall not serve for a period greater than 6 years consecutively, after which time they are not eligible for appointment or reappointment until 3 years have elapsed from any period of previous service. If a successor is appointed to fill the balance of any unexpired term of a member, the time served by the successor shall not apply in computing the 6 years’ consecutive service unless the balance of the unexpired term exceeds 18 months.