How does a woman count the first days when menstruation is abnormal and intermittent, followed by normal menstrual bleeding, given that the total number of days exceeds the usual duration of menstruation?

If the aforementioned blood flows after fifteen days or more since the previous menstruation ceased, then it is menstruation. And if the period of purity (ṭuhr) interrupts the days of menstruation and does not exceed fifteen days, then scholars have two opinions: The Hanbalis and Malikis believe that the woman should perform ghusl (ritual bath), pray, fast, and her husband is permitted to have intercourse with her, as she is considered pure. As for the Shafi'is and Hanafis, they believe that all these days, including the days of bleeding and the intervening purity, are considered menstruation.

If the interruption exceeds fifteen days, then everyone agrees that she is in a state of istihadah (non-menstrual bleeding). The maximum duration of menstruation is fifteen days.