In the first few years, it’s normal for your preteen or teen’s menstrual cycle (the time from day 1 of one period to day 1 of the next period) to vary from 21-45 days. About three years after their first period, they should have a period every 21-34 days, with the average cycle length being 28 days.
The length of a period, (duration), can vary from one person to another, and from one menstrual cycle to another, but the typical length is between 3-7 days of bleeding. For more information about menstruation, visit BLOOM’s Menstruation 101 page.