A “normal” menstrual cycle can vary greatly from woman to woman. The average length of a menstrual cycle is 28 days. Normal variation would include plus or minus 7 days to the average length of a cycle (i.e. a 21 day cycle to a 35 day cycle.) The duration of a menstrual period can vary great, as well. The average duration is 3 days, but normal variation can be as great as seven days. the heaviness of a period differes not only from person to person but with age. Passing large clots or changing pads several times per day usually suggests greater than normal flow and can lead to health complications like low blood count. Among the causes of abnormal uterine bleeding include hormonal irregularities, medicines, pregnancy, stress, benign lesions, and (occasionally) cancers or precancers.