The Official Website for The G.A. Wahid Khorasani

498. Women with the habit of duration are of two types:
1. A woman whose duration of her days of Îayà is the same in two consecutive months, but the time (of the month) she observes blood is not the same. In this case, the number of days she observes blood will be her habitual duration. For example, if she observes the blood of Îayà from the first day to the fifth day in the first month, and from the eleventh day to the fifteenth day in the second month, her habitual duration will be five days.
2. A woman who observes the blood of Îayà for three or more days in two consecutive months, and is purified from it for one or more days, and then observes blood once again, provided the time she observes blood in the first month is different from the second month. In such a case, if the sum of all the days she observed blood and the days that she was purified does not exceed ten days, and it is the same in both the months, then all the days she observed blood and the interval of purity in between will be considered as her habitual duration of ÎayÃ.
It is also not necessary for the days that she was purified between the two discharges to be of the same duration in both the months. For example, she observes blood from the first to the third in the first month, and is purified for two days, and then observes blood once again for three days. In the second month she observes blood from the eleventh to the thirteenth, and is purified from it for two days or more, or less, and then observes blood once again, and the sum of all the days is eight, her habitual duration is eight days.
Another example would be that she observes blood for eight days in one month, and in the second month for four days, after which she is purified, and then observes blood once again, and the sum of all the days of her discharge and the interval of purity is eight days. In this case, her habitual duration will be eight days.

499. If a woman with the habit of duration observes blood with the qualities of Îayà for a duration that is longer or shorter than her habitual duration, but it does not exceed ten days, she should consider all of it as ÎayÃ. If it exceeds ten days, and all of it possesses the qualities of ÎayÃ, she should consider it as Îayà for the length of her habitual duration, starting from the (first) day she observed blood. She should consider the rest as istiÎÁÃah.
However, sometimes all the observed blood is not of uniform quality, in that it possesses the qualities of Îayà during some of the days, and the qualities of istiÎÁÃah during others. In such a case, if the duration of the days the blood possesses the qualities of Îayà coincides with the duration of her habitual days, she should consider it as Îayà during those days, and istiÎÁÃah during the rest.
If however, the days in which the blood possesses the qualities of Îayà are more than her habitual days, she should consider it as Îayà for the length of her habitual days. As for the exceeding days in which the blood possesses the qualities of ÎayÃ, but does not surpass ten days, obligatory precaution dictates that she combine the prohibitions of a ÎÁÞià with the obligations of a mustaÎÁÃah during those days.
If the days in which the blood possesses the qualities of Îayà is not less than three, but it is less than her habitual days, she should consider it to be Îayà during those days. As for the exceeding days, up until the length of her habitual duration, obligatory precaution dictates that she combine the prohibitions of a ÎÁÞià with the obligations of a mustaÎÁÃah during those days.