1. Menstruation (haiḍh) is that blood, which the womb of a mature female excretes and that is not the result of any illness, pregnancy, or that is post-menopausal.

2. Its minimum duration is three days and its maximum is ten days. A day refers to a 24-hour period. Hence, the minimum menstruation period would be 72 hours.
3. Discharge of any color red, yellow, muddy, green, black, or earthy which a woman sees in the days of menstrual bleeding is considered menstrual discharge and her menstruation will continue until the discharge is pure white or there is no discharge. Once blood flows onto the external skin of the vagina (i.e., out of the inner lips), menstruation commences. This is irrespective of whether it flows out to the outer lips or not. If some cotton, a pad, or a tampon is inserted into the vagina whereby the blood cannot flow out, then as long as the blood remains in the vagina and no blood is seen on the outside of the cotton,  menstruation has not commenced. When blood appears onto the inner lips of the vaginal opening (or on the external visible area of the inserted cotton wool), menstruation will commence from the time the blood is seen.
Example: If a woman inserted a piece of cotton or tissue in the internal vagina at night and in the morning she saw blood on the tissue, menstruation will be calculated from the time she saw the blood.

• It is disliked (makrūh) for a woman to use tampons or to insert anything into the inner vagina (Radd al-Muḥtār).

4. The minimum duration of purity (ṭuhr) or duration of cleanliness between two cycles is 15 days. If a woman, after completing her cycle begins to bleed again within 15 days, it will not be considered the start of a new cycle.
5. There is no maximum duration for purity. Hence, a woman will remain pure as long as she does not bleed, even if it extends to several months or years.
6. If a day or more of purity falls within the ten-day period of menstruation, it will be [treated] as [continuously] flowing blood (Qudūrī).
Example: A woman bleeds for three days and then does not see blood for two days. Then, she bleeds for one day; all six days will be considered menstruation.
7. A girl below the age of nine does not menstruate. If such a girl notices blood, it is bleeding due to some illness and will be considered dysfunctional uterine bleeding (istiḥāḍa), not menstruation.
(The Laws of Menstruation, Postnatal Bleeding, & Dysfunctional Uterine Bleeding)
(By Mufti Abdur Rahman Mangera)