According to the Office on Women's Health, the average woman has a period every 28 days. It's not uncommon for the time between periods to vary anywhere between 21 and 35 days.
The Office on Women's Health says that the menstrual cycle, which provides important hormones to the body, starts on the first day of a woman's period. That cycle lasts until the start of her next period. The time of a woman's menstrual cycle varies depending on her age and level of hormones. For example, the average cycle range for adults is between 21 and 35 days; however, the cycle ranges between 21 and 45 days for teenagers.