[What should a woman do if bleeding continues?]

Answer: This woman must look to the time she was accustomed to menstruate and leave prayer and fasting during that time. When the customary time ends, she performs ghusl, fasts, and prays, and everything permitted to the pure becomes permitted to her—entering the mosque, reciting the Qur’an, and so forth.
If the woman forgot the time she used to menstruate, and forgot how many days it lasted, then she should distinguish (tastadill) menstrual blood, for it has signs reported from the Prophet (May Allah bless him and his family and grant them peace): dark color and a distinct odor known to women. When she finds these descriptions, she leaves prayer and fasting and treats it as menses; when they cease, she performs ghusl and prays, etc.
If she cannot distinguish menstrual blood from the blood of istiḥāḍah (non-menstrual bleeding), then she must revert to the habit of her womenfolk; this is on account of the ḥadīth from the Prophet (May Allah bless him and his family and grant them peace) in which he ordered the mustaḥāḍah to do so, saying: “Menstruate as (other) women menstruate.” The scholars understood “women” to mean her female relatives on her father’s side.
What is more persuasive is that she reverts to the prevailing habit among women: that menstruation comes once a month and its number of days is a week. Hence what is related from the Prophet (May Allah bless him and his family and grant them peace) in the ḥadīth of the mustaḥāḍah: “Menstruate six or seven (days).”
Yes, there is no universal habit among women regarding which part of the month the menses begins—whether the beginning, middle, or end. In the ḥadīth of the mustaḥāḍah, the Prophet (May Allah bless him and his family and grant them peace) said to her: “Menstruate six or seven days—as Allah knows.” It may be inferred that she counts six or seven days from whichever part of the month, for the Prophet (May Allah bless him and his family and grant them peace) so commanded her; and there is no way for her to know what is in Allah’s knowledge, and Allah does not charge a soul except [with that within] its capacity. She should also act upon indications to determine the time of menses, even weak ones.
Yes: if blood comes and then continues, she counts seven days from its beginning as menstruation, then performs ghusl; when three weeks pass, she counts menstruation again—and so on.
If the period becomes prolonged for her and the reckoning is confused, let her revert to reasonable judgment and indications. She may remember, for example, seeing the moon in a certain phase or position, and the like; she may take that as the time of menstruation, and so forth.
She should not—and it is not permitted for her to—abandon two pillars of Islam, prayer and fasting, on account of her confusion about their timing and number—especially given what has come in the ḥadīth of the mustaḥāḍah: that she is commanded to pray and to purify herself, that she menstruates six or seven days as Allah knows, and that she menstruates as women menstruate. At most, she may end up praying and reciting the Qur’an while actually menstruating—and that is lighter than abandoning the prayer.
The proof for this is that Allah, Exalted is He, repeats the command to pray in His Noble Book and commands maintaining it, and He has emphasized that to the utmost in numerous verses beyond counting. As for the menstruant’s leaving the prayer, even though it is well known, it is not mentioned in the Qur’an even once; and prayer without ritual purity has been permitted in cases of necessity.
Yes: What appears is that the reason for the menstruant leaving prayer is lack of purity. Thus, if the confused woman counts a week from the beginning of the month, then performs ghusl and prays, her prayer is possibly valid and possibly invalid; the possibility of invalidity is less likely, because the days of purity are, on average, three times the days of menstruation. Therefore, abandoning prayer and fasting given such a possibility is neither fitting nor permitted.
If it be said: “On this basis you must require her to always pray and never treat herself as menstruating.”
We say: Had the command to count days of menses not come from the Prophet (May Allah bless him and his family and grant them peace), we would have required that.
Indeed, the detail we have mentioned is supported by:
1. The ḥadīth of the Prophet (May Allah bless him and his family and grant them peace), transmitted among the Zaydīs and Ahl al-Sunnah;
2. The ease and facilitation inspired by His sayings: “And He has not placed upon you in the religion any hardship.” [Al-Ḥajj:78] “Allah intends for you ease and does not intend for you hardship.” [Al-Baqarah:185] “Allah does not charge a soul except [with that within] its capacity.” [Al-Baqarah:286]
3. Maintaining the prayers and fasting—for “there is no good in a religion without prayer.”
Furthermore, the mustaḥāḍah must perform wuḍūʾ for each prayer; if she is able to perform ghusl for each prayer, that is better; otherwise, one ghusl suffices upon the end of the days of menstruation.
She should pack the outlet of the blood with cotton or the like and bind it firmly, as has come in the ḥadīth of the mustaḥāḍah.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1