Istihaadah means three things: 

1 – The woman had a regular period before she experienced istihaadah. In this case she should refer to the regular period that she used to have, and during that time she should stop praying and she is subject to the rulings on menses, and anything other than that is istihaadah, and she is subject to the rulings on istihaadah at that time. 

For example: A woman used to get her period for 6 days at the beginning of each month, then she began to experience istihaadah and she bleeds all the time. So her period is the first six days of every month, and everything else is istihaadah, because of the hadeeth of ‘Aa’ishah (may Allaah be pleased with her) according to which Faatimah bint Abi Hubaysh said: O Messenger of Allaah, I am experiencing istihaadah and I never become pure; should I stop praying? He said: “No, that is a vein. But stop praying for the number of days that you used to menstruate, then do ghusl and pray.” Narrated by al-Bukhaari. In Saheeh Muslim it says that the Prophet (peace and blessings of Allaah be upon him) said to Umm Habeebah: “Stop praying for the number of days that your menses used to last, then do ghusl and pray.” Based on this, the woman who is experiencing istihaadah should stop praying for the number of days that her period used to last, then she should do ghusl and pray, and not worry about the bleeding at that time. 

2 – If she did not have a regular period before the istihaadah, rather she has been experiencing istihaadah since the first time she got her menses, then she should distinguish between different types of blood. Her menses is when the blood is dark or thick or has a distinctive odour, and she is subject to the rulings on menses at that time. Anything other than that is istihaadah and she comes under the rulings on istihaadah. 

For example: A woman sees blood when she first starts to menstruate, and it is continuous, but for ten days it is dark and the rest of the time it is red. Or she sees that it is thick for ten days and thin for the rest of the time. Or she notices that for ten days it has the odour of menstrual blood and for the rest of the time it has no odour. So her menses is the dark blood in the first case, the thick blood in the second and the blood that has an odour in the third. Everything apart from that is istihaadah, because the Prophet (peace and blessings of Allaah be upon him) said to Faatimah bint Abi Hubaysh: “When it is menstrual blood it is dark and recognizable, so when it  is like that, then stop praying, and when it is otherwise, do wudoo’ and pray, for that is from a vein.” Narrated by Abu Dawood and al-Nasaa’i; classed as saheeh by Ibn Hibbaan and al-Haakim. Although the isnaad and text of this hadeeth are subject to further discussion, the scholars (may Allaah have mercy on them) followed it, and that is better than trying to make her follow the pattern of the majority of women.  

3 – If she does not have a regular period and cannot distinguish it properly because the istihaadah is ongoing from the first time she saw the blood, and her blood is all the same or its characteristics are varied but it cannot be menstrual blood. This woman should act on the basis of what is the usual pattern among women, so her menses is six or seven days each month, and she should start from the first time when she saw blood; everything apart from that is istihaadah. 

For example: if the first time she saw blood was on the fifth of the month, and it continued with no change to distinguish menstrual bleeding by colour etc, then her period is six or seven days of each month starting from the fifth of the month, because of the hadeeth of Hamnah bint Jahsh (may Allaah be pleased with her) who said: “O Messenger of Allaah, I bleed a great deal all the time. What do you think – should I stop praying and fasting?” He said: “I suggest you use a piece of cotton, for it will absorb the blood.” She said: It is more than that. In this report he said: “That is a kick from the shaytaan, so count your menses as six or seven days, which is something between you and Allaah, then wash yourself and when you see that you have become pure and you are certain of it, then pray for twenty-four or twenty-three days, and fast.” This hadeeth was narrated by Ahmad and Abu Dawood, and narrated and classed as saheeh by al-Tirmidhi. It was narrated that Ahmad classed it as saheeh and that al-Bukhaari classed it as hasan. 

The words of the Prophet (peace and blessings of Allaah be upon him), “six or even days” does not mean that one has the choice, rather it is for the purpose of ijtihaad, so the woman should look at those whose situation is closest to hers in terms of physical resemblance, age and relationship, etc., and see whose cycle most closely resembles hers. If the closest one has a six-day period then she should regard her menses as six days, and if the closest is seven days then she should regards her menses as seven days. End quote. 

Risaalah fi’l-Dima’ al-Tabee’iyyah li’l-Nisa’ by Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) 

At the time when she decides that the blood is menstrual blood, then she is menstruating, and at the time when she decides that the period has ended, then she is pure and she should pray and fast, and may have intercourse with her husband.