What's a normal menstrual cycle and how often must it be normal?

A range of normal: A menstrual cycle is the time from the start of one menses to the next. It can vary from 21 to 35 days, but most women are around 28-30 days. The menses itself usually lasts 3-5 days, but up to 7 is normal. It doesn't have to be exactly the same number of days each month, but if you are having a wide range of cycle days or bleeding between, you may have some issue with ovulation (egg production.