Dr. Su`aad Salih, professor of Jurisprudence at Al-Azhar University, states the following: “Women may be classified into the following categories as to different phases of their menstrual cycle:

1. Regular menses: This is the case when a woman has a normal period, in the sense that there is no irregularity as for the time she gets her menses and its duration, which lasts normally for 5 or 6 days.

2. Extended period: This is the case when a woman’s period exceeds the normal time, in the sense that she has moved to the stage of istihadah (chronic blood discharge).

3. Confusing menstruation with chronic blood discharge.

4. Perplexity over the duration of menstrual period: This is the case when a woman is unaware of the time and the lasting of her menstrual period. She is called ‘the perplexed’ or ‘the confused’.

Menstruation begins at the onset of any of its signs that may appear in any color so long as it is the due time for it. It ends by the moment purification is attained. There are many signs for this. Among these signs is the appearance of the white fluid discharged after the bleeding. Another sign is dryness which involves that the towel or the piece of cotton that is applied on woman’s vagina becomes dry and no more blood oozes for a day and night. Once these two signs occur, a woman is to perform ghusl (purifactory bath) and resumes performing prayer. Any discharge of blood that comes after that is considered to be istihadah, for which a woman is just to make ablution before every prayer and apply a sanitary towel.”
So, in the light of the above, you stop performing prayer as soon as you get the afore-mentioned sign of onset of menses, and this sign is further clarified in the following words of Sheikh Sayed Sabiq, quoted from his book Fiqh As-Sunnah:
“There are some requirements for blood to be regarded as menstrual blood:
1. Dark: It is reported that when Fatimah bint Abi Hubaysh had a chronic discharge of blood, she asked the Prophet (peace and blessings be upon him) about this and he said to her: ‘If it is menstrual blood, it is dark and distinguishable. If it is so, then stop performing prayer. If not, then make ablution and pray, for it is only due to a vein (i.e., it does not require taking a purificatory bath.” (Reported by Abu Dawud, An-Nasa’i, Ibn Hibbaan and Ad-Daraqutni)
2. Red: It is the original color of blood.
3. Yellow: It is a liquid that looks like pus.
4. Muddy: It is an intermediate between black and white, like dirt.
`Alqamah Ibn Abi `Alqamah q
uoted his mother, Murjaanah, the freed maid of `A’ishah (may Allah be pleased with her) as saying: ‘Women would send `A’ishah small boxes with yellow-stained cotton, and she would tell them, ‘Do not be in haste until you see the pure white cotton.'” (Reported by Malik, Muhammad Ibn Al-Hasan and Al-Bukhari)
If the discharge is yellow or muddy during the days of menstruation, it is to be considered part of the menses, whereas it is not regarded as such during other days. Umm `Attiyyah narrated: ‘On getting clean, we would never consider the yellow or muddy discharge to be anything.’ (Reported by Abu Dawud and Al-Bukhari)
As for its duration, there is no stated minimum or maximum length of time for a menstrual period. All statements dealing with this issue have no sound backing. If a woman has a customary length of time for her period, she should act according to it. This is based on the hadith reported by Umm Salamah when she asked the Prophet (peace and blessings be upon him) about a woman who has a chronic blood discharge. He said, ‘She should look for the number of days and nights that she usually has her period and the time of the month during which it occurs. Then she should stop praying, (during these days, and then afterwards) perform ghusl , put something over her vagina and then perform prayer.”