How can the duration of the menstrual cycle be accurately determined, based on the description that includes the cessation and return of blood, and the appearance of a brownish or yellowish discharge, given that clear blood is often not seen after the seventh or eighth day?

When a woman sees blood, she must refrain from prayer and fasting, and she should perform ghusl (ritual bath) and pray after its cessation. If the blood returns during the period of menstruation, it is considered menstruation, even if it is pinkish. If the blood stops again and she sees purity by dryness or by the white discharge (al-qassah al-bayda’), she must perform ghusl. Some scholars are of the view that if a woman sees purity by dryness during her customary period, she may delay ghusl for a short while, but it is more cautious to hasten it. As for yellowish or brownish discharge (as-sufrah wal-kudrah), that which is connected to blood is menstruation, and that which is not connected to it is not menstruation, unless it occurs during the customary period. If she experiences yellowish or brownish discharge again after achieving purity, it is not considered menstruation unless it occurs during the customary period.