Typically, the menstrual cycle will last for 28 days, which is calculated from day one of a menstrual period to day one of the next cycle. Ovulation typically occurs 14 days before the start of your menstrual period. If your cycle lasts for 28 days, you can ovulate on the 14th day like the average female. However, you will start ovulating around the 11th day if you have periods 25 days apart. Alternatively, you will not ovulate until the 20th day if your cycles are longer, like for 34 days. If periods are irregular it would be more challenging to determine the fertile days.