A normal menstrual cycle could be as short as 22 days or as long as 36 days. When doctors talk about the length of your cycle, they include the days when you have your period. If you have a regular 28-day cycle you will start your period every four weeks. If you have a 22-day cycle you’ll have a period about every three weeks. The average period lasts for three to four days, but it can be shorter or longer, ranging from between one and seven days.