Women having the habit of fixed duration
498. Women having the habit of duration are of three types:
acts which are forbidden on Haa’ez and should perform her obligations and offer her prayers. However, if the duration in two consecutive months, during which she is Pak, is not the same, there is an objection (Ishkaal) in classifying such woman as the one having the habit of fixed duration. However, such woman should act according to the precautionary measures, i.e. during the days when she is Pak, she should refrain from the acts which are forbidden on Haa’ez and should perform her obligations and offer her prayers, and during the days when she is bleeding, she should refrain from the acts which are forbidden for Haa'ez and should perform the obligations of a Mustahadha.
499. If a woman with the fixed habit of duration sees blood for more days than her habitual duration, and if it exceeds 10 days and the nature of blood remains same throughout, then she can calculate her habitual duration from the day bleeding began or from the middle, and treat it as Hayz and rest as Istihadha. But if the nature of blood changes, with some days showing signs of Hayz and others showing signs of Istihadha, then there can be three possibilities:
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 Istihadha.
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 Istihadha.
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 Istihadha.