What is the number that is considered menstruation if its duration varied between 8 and 9 days in consecutive months?

The preponderant view is that all blood seen by a woman during a possible period is considered menses, even if the customary period comes early or late, or increases or decreases in duration. This is what the narrations indicate and what is apparent from the actions of women during the time of prophethood, as no mention of a customary period was transmitted from them, nor was any clarification concerning it transmitted from the Prophet, peace and blessings be upon him, except for women experiencing Istihada (non-menstrual vaginal bleeding). Thus, a woman abstains whenever she sees blood that could be menses, whether it coincides with her custom or deviates from it. As for the view that requires three repetitions to establish that what deviates from the custom is menses – whereby a woman fasts and prays during this period until the blood recurs three times, and then if it recurs, it becomes a custom and she makes up the obligatory fasts she observed – it is a weak (marjuh) opinion.