The usual duration of menstruation is six or seven days. The evidence for this is the Prophet’s ﷺ statement: “Observe menstruation, according to Allah’s knowledge, for six or seven days.” (1)

When it is impossible to know a particular matter precisely, one refers back to its general category. Since this woman cannot determine her own menstruation specifically, she refers to other women of her kind.

The stronger view is that she should refer to the usual pattern of her female relatives, such as her sister, her mother, and the like, rather than to the pattern prevailing among menstruating women generally. A woman’s resemblance to her relatives is closer than her resemblance to women in general.

His statement, “of every month,” is because most women menstruate once a month.

The evidence for this is Allah’s saying:

“Divorced women shall wait by themselves for three menstrual cycles.” [al-Baqarah 2:228]

And Allah says:

“As for those of your women who have despaired of menstruation, if you are in doubt, their waiting period is three months, as is that of those who have not menstruated.” [al-Talaq 65:4]

Thus, Allah has assigned one month to each menstrual period, and this is what generally occurs.

She begins counting the month from the first bleeding she experiences. Thus, if the first day on which the blood came to her was the fifteenth, she begins from the fifteenth. If we say seven days, then it extends until the twenty-second; and if we say six, then until the twenty-first; and so on.

If she has forgotten and does not know whether her menstruation came at the beginning of the month, on the tenth, or on the twentieth, then she should count it from the beginning of the month as a precaution.