It is possible that a girl's period will not come on a regular basis during the first few years after she begins her period. At first, this is typical. A girl's menstruation should occur once every 4-5 weeks by roughly 2-3 years following her first period. However, if you are not getting periods regularly, then this could be a sign that you are not developing properly and need medical attention. The most common cause of delayed menarche is poor nutrition or eating disorders such as anorexia or bulimia. If you are concerned about how soon you're going to start menstruating, ask your parent's or guardian's about their experience with girls your age. They may be able to provide you with some insight into how quickly you're expected to begin menstruating. If you aren't getting your periods regularly, see your doctor to determine the cause of your problem. Your health depends on it!