Hi, I have gone through your query and here are your answers and some suggestions: 1. During the initial years after puberty, there is some irregularity in the menstrual cycles as the body tries to adjust the balance. This usually settles down in a few years.2. If you maintain a healthy body weight, exercise regularly and avoid stress and pollution, normal hormone milieu is restored in usual circumstances. 3. Eating habits, body weight changes, stress and modern lifestyle affect the hormone metabolism and thereby the regularity of the cycles. 4. To know when your period is over, you need observation of a few cycles and note your pattern. Each woman has her own pattern, but, on the average a period lasts for 3-7 days. 5. As regularity sets in, you can predict your period with accuracy. You can check the premenstrual symptoms, consistency of the vaginal discharge etc. Hope this helps.