Don't worry if your early periods have longer cycles or don't follow a schedule. This irregularity is normal for at least the first 2 years. Your periods should become more regular within two years after you start menstruating. Some teens have a 28-day cycle; some have a 24-day cycle; others have a 30- to 34-day cycle. All of these are normal. For young teens, cycles can range from 21 to 45 days. For adults, it can be 21-35 days. If your period is much shorter or longer, or if your period does not become regular after two years, see your primary health care provider.