Menstruation starts when a girl reaches puberty. When a girl menstruates, blood is discharged from her vagina and this usually lasts 3 to 5 days. The amount of time between a girl's menstruation is known as her menstrual cycle. Usually a menstrual cycle lasts for 28 days. for girls who are just starting to menstruate, the cycle is often not regular. after a year or two, the menstrual cycle will become more regular.