The period between the start of one menstruation to the start of the next menstrual bleeding, which usually lasts anywhere from 21-35 days, is called menstrual cycle.
If your first menstrual day is on the first day of February and the next menstrual period begins on 29 February, your menstrual cycle was 28 days long. If your period starts on February 1 and the next one starts on March 5, your cycle was 33 days long.
So the length of the cycle varies in different people. It may not be the same in even one individual, from month to month . It can also change due to various factors such as anxiety, illness, climatic changes, travel, nutrition, and so on.
Again, bear in mind that it may take up to two years from your first menstruation until the menstrual cycle is regulated.
Remember: you need to mark your first menstrual day on the calendar each month.