This may come as a surprise, but your 'monthly' cycle doesn’t necessarily take place once a month. The average menstrual cycle time for most girls is 28 days, but your cycle may last from 21 to 35 days and still be normal. In your cycle, 'day one' is the first day of your period, or the first day you begin to bleed.
If you have a short cycle, you may have a period more often than once a month. However, if your cycle lasts longer, you’re one of the girls who have fewer periods in a year.
In general, a girl's period will start somewhere between the ages of 10 and 13 (on average at age 12). It may take a year or two for a girl’s period to become regular. She will have a period about once a month (except if she is pregnant) until she reaches menopause, the time when her periods stop, typically somewhere around age 51.