It is a monthly bleeding to which all healthy women are subjected during their child bearing years. Its duration is according to the norm for each individual woman ranging from 1-15 days (according to fuqaha).
However according to Sheikh Uthaymeen though, Read it HERE
“Menstruation (al-hayd): Allaah has attached numerous rulings to it in the Qur’aan and Sunnah, but He did not state the shortest or longest length (of a menstrual period), or the length of the period of tahaarah between two menstrual periods, even though the need to know that exists.”
As a young lady first getting your menses you should observe your own blood flow, duration, frequency and pattern, body pain and so forth. The menses will be your normal flow with slight hiccup now and then. However as you aged, the pattern will change.
More information on Menstruation HERE
This is the bleeding that occurs following the delievery of a child and maximum duration is 40 days.
It is reported on the authority of Umm Salamah that she said:
“The woman with nifaas used to be in confinement for forty days during the lifetime of the Messenger of Allah sallallahu alaihi wasalam. (Abu Dawood, Ibn Majah and Tirmidhi)
If the bleeding has stopped before 40 days, then she must make ghusl. If it exceeds more than 40 days, she must still make ghusl. Read fatawa by ISLAM QA HERE
Forbidden Acts for the Menstruating or Post-Childbirth Woman
1. Fasting. She must makeup for the obligatory fast that she missed during this time
Al-Bukhaari (321) and Muslim (335) narrated that Mu’aadhah said: I asked ‘Aa’ishah:
“Why does a woman who menstruates have to make up the fasts but not the prayers?” She said: “Are you a Haroori?” I said: “I am not a Haroori, but I am asking.” She said: “That used to happen to us and we were commanded to make up the fasts but we were not commanded to make up the prayers.”
Al-Nawawi (may Allaah have mercy on him) said:
This ruling is agreed upon. The Muslims are unanimously agreed that menstruating women and women who bleed following childbirth are not obliged to pray or fast at that time, and they are agreed that they do not have to make up the prayers, and they are agreed that they have to make up the fasts. The scholars said: the difference between them is that the prayers are many and repeated, and it would be too difficult to make them up, unlike the fast which is enjoined only once in the year, and a woman’s period may last only one or two days.
2. Sexual intercourse.
Allah says in Surah Baqarah 2: 222
They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have a sexual intercourse with his wife while she is having her menses), therefore keep away from women during menses and go not unto them till they are purified (from menses and have taken a bath). And when they have purified themselves, then go in unto them as Allâh has ordained for you (go in unto them in any manner as long as it is in their vagina). Truly, Allâh loves those who turn unto Him in repentance and loves those who purify themselves (by taking a bath and cleaning and washing thoroughly their private parts, bodies, for their prayers). (222)
If one does sexual intercourse during the woman’s menses
1. Both of them must make repentance
2. Even though there are difference of opinion on this, they must make an expiation.
Ibn Abbas said that which states the the Prophet sallallahu alaihi wasalam said regarding the one who has sexual intercourse with his wife during her menses: “He should give a dinar or half dinar as charity” (Abu Dawood, Nasaai and Ibn Majah)