See a picture of a woman's reproductive system. The menstrual cycle is measured from the first day of menstrual bleeding, Day 1, up to Day 1 of your next menstrual bleeding. Although 28 days is the average cycle length, it is normal to have a cycle that is shorter or longer.1  A teen's cycles may be long (up to 45 days), growing shorter over several years.enstrual period. On Day 1 of your cycle, the thickened lining (endometrium) of the uterus begins to shed. You know this as menstrual bleeding from the vagina. A normal menstrual period can last 4 to 6 days. Most of your menstrual blood loss happens during the first 3 days.