The menstrual cycle is counted from the first day of the period to the first day of the next period. It is a range of anywhere between 21 -45 days for teenagers and younger women. For adult women the range is usually between 28-35 days. Many factors can affect the menstrual period, including body weight, stress, thyroid or other hormonal problems, excessive/extreme exercise, and medications (e.g. antidepressants).
Some women may also experience excessively heavy and painful periods and may pass large blood clots or require extra sanitary pads to prevent overflow. This needs to be investigated to rule out problems like fibroids or endometriosis.