Periods or menstruation usually begin during puberty, between 10 and 16 years of age, and they continue until menopause. The interval between a woman's two menstrual cycles is usually 28 days, but it varies slightly in every woman. In some women, this period can range from 21 days to 40 days. Most women usually have about 11 to 13 menstruations or periods in a year. When menstruation
first begins, it may take up to 2 years to establish a regular cycle. This means that after the first period, it takes two years for the periods to become regular.