What is the method for establishing the menses duration for a woman if scholars prescribe a fixed period of six or seven days without repetition?

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For those who mandate sitting for six or seven days, that duration is observed immediately without the requirement of repetition. Any amount exceeding that established period is not considered menses until it recurs. Similarly, if the duration is determined by the established custom of her female relatives (adat an-nisa'), she observes that duration without repetition.