If she asks about bleeding that continues beyond fifteen days, the answer is: You are experiencing irregular bleeding, so do not refrain from prayer for this entire period. Bleeding that is not menstruation—such as bleeding lasting less than a day and a night, or bleeding during pregnancy—is not irregular bleeding, but it is subject to the rulings of irregular bleeding. Some jurists call it corrupt blood. (1)

The sound view in this matter as well is that there is no maximum limit. Some women have an established, regular cycle of seventeen days, or sixteen days. So what makes the blood before sunset on the fifteenth day menstruation, while blood occurring one minute after sunset is irregular bleeding, when its nature, colour, and flow are all the same? How can it be said that, with the passing of one or two minutes, the blood has changed from menstruation to irregular bleeding without any evidence? Had there been evidence for what they say, we would have accepted it.

Thus, if she has a continuous, established cycle of—for example—seventeen days, we say that all of it is menstruation.

But if her bleeding continues throughout the entire month, or stops only briefly, such as for a day or two, or if it is intermittent—coming for a few hours and then ceasing for a few hours throughout the month—then she is experiencing irregular bleeding. In that case, we treat her according to the rulings governing one who has irregular bleeding, as will come, if Allah Most High wills. (2)

“Its usual duration is six or seven, and the shortest period of purity between two menstrual periods is thirteen days …”

His statement, “Its usual duration is six or seven,” means that menstruation most commonly lasts six or seven nights.