The Center for Population Research collected 30,655 recorded menstrual cycles from 2,316 women. Ninety-five percent of all cycles were between 15 and 45 days long. The mean and standard deviations for the total number of cycles were 29.1 and 7.46, respectively. Cycle lengths between 15 and 45 days averaged 28.1 days with a standard deviation of 3.95 days. Variability of menstrual-cycle lengths is highest for women under 25 years of age and declines steadily to reach a minimum for ages 35 to 39. It is then followed by a slight increase for women aged 40 to 44. The pattern of variability by age is demonstrated equally well by the percent of cycles between 25 and 31 days in length as by the standard deviation.
Chiazze L, Brayer FT, Macisco JJ, Parker MP, Duffy BJ. The Length and Variability of the Human Menstrual Cycle. JAMA. 1968;203(6):377–380. doi:10.1001/jama.1968.03140060001001