If the bleeding lasts all month then she is experiencing istihaadah (irregular non-menstrual vaginal bleeding) and she should refer to her regular cycle if she used to have a regular cycle before that, and stop praying etc during the time that was her regular period, then she should do ghusl and pray and fast for the rest of the month even if she is still bleeding. If she did not have a regular cycle or she has forgotten it, then she should distinguish the types of blood if she can. Menstrual blood (hayd) and non-menstrual blood (istihaadah) may be distinguished by colour, smell and thickness; menstrual blood is black or dark, and has an unpleasant smell, and it is thick, unlike the blood of istihaadah.
The days on which the blood has the characteristics of menstrual blood are menses, and on the other days she is regarded as “pure”, so she should pray and fast.
If she cannot distinguish different types of blood, then she should stop praying etc for six or seven days, because that is the usual length of menses for women, then she should do ghusl and pray and fast.
Among the reports of the Sunnah which speak of istihaadah and say that the woman should refer to her previous cycle – if she used to have a known cycle – is the report narrated by al-Bukhaari (319) from ‘Aa’ishah (may Allaah be pleased with her), that Faatimah bint Abi Hubaysh asked the Prophet (peace and blessings of Allaah be upon him): I suffer from istihaadah and I do not become pure; should I stop praying? He said: No, that is just a vein. But stop praying for the number of days that you used to menstruate, then do ghusl and pray.”
Among the reports that speak of differentiating between types of blood is that narrated by al-Nasaa'i (215) and Abu Dawood (304) from Faatimah bint Abi Hubaysh, that she suffered from istihaadah and the Messenger of Allaah (peace and blessings of Allaah be upon him) said to her: “If it is menstrual blood then it is black and recognizable, so refrain from praying, then when it is the other type, do wudoo’, for that is from a vein.” Classed as saheeh by al-Albaani in Saheeh al-Nasaa’i.
Among the reports which speak of the woman who has istihaadah stopping prayer for six or seven days if she did not have a regular cycle or she cannot distinguish between types of blood is the report narrated by al-Tirmidhi (128) and Abu Dawood (287) from Hamnah bint Jahsh who said: I used to suffer istihaadah, bleeding heavily. I came to the Prophet (peace and blessings of Allaah be upon him) to ask him and tell him about that. I said: O Messenger of Allaah, I suffer istihaadah, bleeding heavily. What do you command me to do? It is keeping me from fasting and praying. He said: “That is only 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 and pray. That will suffice you, so do what other women do with regard to their menses and purification.”
Al-Tirmidhi said: I asked Muhammad [al-Bukhaari] about this hadeeth and he said: It is a hasan saheeh hadeeth. Ahmad ibn Hanbal said likewise, it is a hasan saheeh hadeeth.
And Allaah knows best.