The average duration of the menstrual cycle is 28 days, which represents the typical length from the first day of one menstrual period to the first day of the next. This cycle length allows for the processes of follicular development, ovulation, and endometrial preparation necessary for potential implantation. While individual variation exists, 28 days is considered the standard for a normal menstrual cycle. Understanding this duration is crucial for timing ovulation and managing reproductive health.

Reference: Williams Gynecology, Barbara L. Hoffman et al., 3rd Edition.