According to the Cleveland Clinic, the average woman's period lasts for 3 to 5 days. However, anywhere from 2 to 7 days is within the normal range.
The Clinic explains that prior to menstruation, the lining of the uterus has prepared for the implantation of a fertilized egg. When that does not occur, the extra layer of lining is shed, and the characteristic bleeding occurs. As soon as menstruation has ended, the additional lining starts to develop again. About halfway through a woman's 28-day cycle, an egg is released from an ovary during ovulation. If the egg is not fertilized by sperm, the woman's period begins again.