There is no set minimum limit for the period of purity between two menstrual periods, rather when a woman sees the blood – i.e., menstrual blood which is “black” (dark red), recognizable and has a distinct odour – she should refrain from fasting and praying, on condition that the total number of days of menstrual bleeding does not exceed half of the cycle (i.e., fifteen days in the month).
Yellowish and brownish discharges seen after the period has ended are of no significance and they do not come under the same rulings as menstruation.
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And Allah knows best.
