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Salaamun Alaykum

Ruling no 499: If a woman with the fixed habit of duration sees blood for less or more days than her habitual duration, but the number of those days does not exceed 10 she should treat them as Hayz. And if it exceeds 10 days and the nature of blood remains same throughout, then she will calculate her habitual duration from the day bleeding began, and treat it as Hayz. But if the nature of blood changes, with some days showing signs of Hayz and others showing signs of Istihaza, then there can be three possibilities:

a) If the number of days in which blood shows signs of Hayz tallies with the habitual duration, then she will take those days as of Hayz, and the rest as Istihaza.

b) If the number of days in which blood shows signs of Hayz exceeds her habitual duration, then she will take her habitual duration as Hayz, and the rest as Istihaza.

c) If the number of days in which blood shows signs of Hayz is less than her habitual duration, she will add some days to complete her duration and take that period as Hayz, and treat the rest of the days as of Istihaza.

(Ayatullah Sistani’s Islamic Laws)
Laila Habib