Q: How do I determine my regular monthly menstrual cycle?
A: First it should be noted that age of menstruation is from a complete nine hijra years of age to a full 50 years of age. Any blood that appears before or after this period is not considered menstrual blood.
The shortest menstrual cycle is one day and night, the longest is 15 days and nights, and the average is 6-7 days.
The shortest duration between cycles is 13 days. There is no limit to how long the duration between cycles can be.
To determine your regular cycle (for first-timers 9 years of age or older) you are to stop praying etc at the first sight of blood. If it continues for a complete day and night it is considered menstrual blood.
At this point you are to perform ghusl regardless if the blood has stopped or not and resume prayer etc. Because you do not know your regular cycle, any blood which flows after the initial day and night could potentially be pseudo-menstration which does not prevent prayer nor require ghusl.
If the blood flow does not exceed 15 days it is considered menstrual blood. You are to perform ghusl again and make note of your first month's cycle duration.
This process is repeated for another 2 cycles (totaling 3 cycles). If the duration of each cycle's blood flow has been consistent you can determine you regular menstrual cycle. This regular cycle will now be considered in the following month.
Source: Sharh al-Muntaha