Your first period may last from two to seven days. Then, there might be 21 to 40 days or even longer before you have another period. Your next period might be heavier or lighter than the first. 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 two 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. It is possible to skip a month, especially if you have been ill or under stress.