Men often assume the term “menstrual cycle” means menstruation or when a woman is having her period. They do not realize that a woman’s period or menstruation, which normally lasts for 1 to 8 days (3 to 5 days is most common), is only one part of her menstrual cycle, which normally lasts for 21 to 35 days. See Menstrual Cycle and Menstual Flow