To Mimic Nature
The predominant pattern of natural menses
occurs on a 28 day cycle.
(although some cycle on shorter or longer intervals) the bcp with either 21 active/pack with 7 days off or 21active +7 dummy pills in a 28 day pack for daily use is an industry standard. The 7 days off allows menstral flow, some give continuous active pills for 3-4 mo or more & decrease # of menses they have /yr.