The duration of the female cycle is very variable. It varies from woman to woman. It varies within each woman during a particular time frame and it varies with age.
A variable cycle duration within the same time frame within the same woman can be explained by natural fluctuations but also be associated to situative circumstances such as physical or psychological stressors.
Changes with age are characterized by most regular cycle durations between 25 and 40 years, and least regular cycle durations at the very beginning and end of the fertile life period.
During puberty, cycles are often longer, cycles prior to menopause are often shorter compared to other time periods within the female fertile period.