While every woman is different, the range for a normal menstrual cycle lasts 24-35 days. These days are counted from the first day of bleeding (Day 1 of the cycle), to the day prior to the next period. On average, most women have a cycle of 28 days.
The menstrual flow (period) typically lasts between 3-7 days, with bleeding heaviest during the first two days.
Some people have a regular menstrual cycle, meaning that its length is fairly uniform from one month to the next, and that what happens during that cycle is also regular for the different months. For example, the flow is light or moderate, and that is the norm for that individual. However, some women, due to various factors, have an irregular period. For example, the cycle may last 25 days in one month, drop to 19 days in the next month, and then run for 34 days in the next month. This is irregular because the variation from one month to the next is glaringly noticeable.
Minor variations like having a cycle run for 28 days in one month, then 25 the next, and 27 during the third month may not qualify as an irregular cycle since the changes are within a narrow range. You aren’t a machine after all that must always run like clockwork, so tiny changes like that shouldn’t overly concern you.
Since everyone is different, we recommend that you track your own cycle and determine what is normal for you, so that once you observe significant changes to your “norm” you can contact a professional for advice regarding whether you should be concerned or no action is warranted regarding those observed changes.