A menstruation cycle, if regular, begins on the first day of bleeding and ends on the first day of your next bleeding. The cycle can last between 21 and 35 days but the average cycle is about 28 days. The length of the cycle is determined by the hormonal changes that take place during each phase of the cycle. A woman's cycle length may vary over the course of her reproductive years. Ovulation, which is when the egg leaves the ovary, usually occurs mid-cycle at about day 14 depending on the length of the entire cycle.