Praise be to Allah.
The scholars (may Allah have mercy on them) differed concerning the maximum length of menses. The view of the majority is that the maximum duration of menses is fifteen days, and anything beyond fifteen days is regarded as istihaadah (non-menstrual vaginal bleeding).
The majority of scholars quoted as evidence for that what is usually the case. What is usually the case is that a woman’s period does not last for more than fifteen days.
Ibn Qudaamah (may Allah have mercy on him) said:
In our view, menses is mentioned in general terms in Islam, without defining a limit to it. Therefore we may say that no limit is specified for it from either a linguistic point of view or from a shar‘i point of view. So with regard to this matter, we must refer to what is normally the case.
‘Ata’ said: I have seen some women whose menses lasts one day, and others whose menses lasts fifteen days.
Ahmad said: Yahya ibn Adam told me: I heard Shareek say: Among us there is a woman who regularly menstruates for fifteen days out of every month. End quote from al-Mughni (1/225)
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
The words “the maximum duration is fifteen days” refer to the maximum duration of menses.
They quoted as evidence for that what is usually the case, because what is usually the case is that a woman’s menses does not last for more than fifteen days.
Moreover, anything more than that would mean that the greater portion of the month was taken up by menses, and it is not possible that the time of tuhr (purity) would last for a shorter time than the time of menses.
So if the menses is sixteen days, then the tuhr (period of purity) would be fourteen days, and it not possible that the bleeding could last longer than the period of purity.
According to the scholars, if a woman’s bleeding becomes constant and never ceases, then she is mustahaadah (suffering irregular, non-menstrual bleeding), so the greater part of the month comes under the ruling on the entire month, and anything longer than fifteen days is regarded as istihaadah. So if any woman’s bleeding lasts for longer than fifteen days, then it is istihaadah.
End quote from ash-Sharh al-Mumti‘ (1/471)
The scholars differed concerning the minimum duration of the period of purity (tuhr) between two menses. We have explained previously in the answer to question no. 221997 that there is no minimum for this period.
As for the maximum length of the period of purity between two menses, there is also no maximum either, according to scholarly consensus.
It says in Adwa’ al-Bayaan by Shaykh Muhammad al-Ameen ash-Shinqeeti (may Allah have mercy on him):
We have stated previously on several occasions that there is no maximum for the period of purity (between menses), according to scholarly consensus. An-Nawawi said in Sharh al-Muhadhdhab: The evidence for scholarly consensus is based on what is usually the case, because it is something that is commonly experienced.
One of the amazing stories concerning this issue is what al-Qaadi Abu’t-Tayyib narrated in his comment. He said: A woman told me that her sister menstruated for only one day and night out of every year, and she is healthy and is able to get pregnant and give birth, and her nifaas (post-partum bleeding) lasts for forty days. End quote.
It says in Nayl al-Ma’aarib bi Sharh Daleel at-Taalib (1/105):
“There is no maximum” means there is no maximum length of the period of purity between two menses, because there is no report of any such maximum in Islamic teaching, and because there are some women who may remain pure (free of menses) for a month or for three months or a year, or for longer than that, and there are some women who do not menstruate at all. End quote.
And Allah knows best.