Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Responding to your question, Dr. Hatem Al-Hajj, Dean of Shari`ah Academy of America, states:
The issues of menstruation (hayd) are controversial, and I will attempt to give you the stronger answer in my own judgment.
Hayd is the natural flow of blood from the womb occurring approximately monthly in post-menarcheal women. If the bleeding exceeds the longest duration of hayd, it is considered istihadah, which is irregular vaginal bleeding, that is caused by a vein called al-`adhil or some other reason.
It appears from the above that we need first to determine the longest duration of hayd. This is widely controversial, and the Hanafi position of ten days seems to be the strongest.
Then, based on our selection of the Hanafi position here, if your bleeding lasts more than ten days, you will not consider it hayd. (Post-partum bleeding is different and may last up to sixty days). If a woman had regular periods, and then they became irregular and exceeded the longest duration of hayd, or if she was never regular and now her periods are longer than the longest hayd duration, she will belong to one of the following three categories:
1- She knows the difference between the menstrual and non-menstrual blood.
2- She can’t distinguish the two types from each other, nor can an expert do that, but she remembers her previous habit.
3- She can neither sort the two types of blood out (or they happened to be one type), nor can she remember her previous habit (or she didn’t have one).
4- If she knows the difference between the menstrual blood – which is black (dark red) in color, thick and heavy in consistency, with a distinct smell, and doesn’t clot – then she will consider that bleeding her hayd and the other type – which is lighter and thinner – her period of purity (tuhr), which should not be less than fifteen days according to the strongest position, which is the majority position as well.
This is because when Fatimah bint Abi Hubaish (may Allah be pleased with her) had a prolonged flow of blood, the Prophet (peace and blessings be upon him) told her, “If it is the blood of menstruation, it will be dark and recognizable. If it is that, then leave the prayer. If it is other than that, then make ablution and pray, for it is only due to a vein.” (An-Nasa’i)
1- If she can’t distinguish the two types from each other, nor can an expert do that, then she will try to depend on her previous habit, and avoid praying and fasting of every month during the days she used to have hayd, and then resume her acts of worship.
This is because `A’ishah, the wife of the Prophet (peace and blessings be upon him), said: “Umm Habiba bint Jahsh complained to the Messenger of Allah about excessive bleeding. He said to her: “Remain away from prayer equal to the length of time that your menstruation holds you back. After this, bathe yourself and pray.” (Al-Bukhari)
2. If she can neither sort the two types of blood out (or they happened to be one type), nor can she remember her previous habit (or she didn’t have one), then she may consider her bleeding as hayd for six to seven days, and fast and pray for the rest of the month.
This is because the Prophet (peace and blessings be upon him) said to Hamnah bint Jahsh: “Observe your menses for six or seven days, Allah alone knows which it should be; then wash. And when you see that you are purified and quite clean, pray for twenty three or twenty four days and nights and fast, for that will be enough for you, and do so every month, just as women menstruate and are purified at the time of their menstruation and their purification.” (Abu Dawud)
1- If you have dark thick blood for a certain number of days, and then it changes, your hayd is only during the time of that dark and thick blood.
2- If you can’t tell the difference between the bloods, go back to your habit, and consider your hayd to be four, five or six days according to your previous habit.
3- If you had no habit, then consider your hayd to be six or seven days and the rest of the bleeding will be istihadah. During the time of istihadah, you can fast and pray, however with a fresh wudu’ for every obligatory prayer and its attached sunnah prayers. You may also have all forms of conjugal relations with your husband during the time of istihadah.
4- If you decided to consider your period 8 days because that was your previous habit, then you start the count from the time your period used to start, let us say Ramadan 4th and by the end of the 8 days, which will be Ramadan 12th, you will consider yourself having irregular bleeding (istihadah) until the same start day of the next lunar calendar month, which will be Shawaal 4th.
During that time, apply the rules of istihadah mentioned here above. As for your 8 days of hayd, from Ramadan fourth to twelfth, if they are interrupted by periods of purity, then consider yourself in the interim pure and pray and fast and do like women do during the time of purity. This means you may fast Ramadan 6th if you were pure from dawn to sunset, and you will not need to make up the fast later. Although the combination of rulings mentioned in this last paragraph would be a minority position, it seems to be the strongest one.
Allah Almighty knows best.