A woman who’s period lasts longer than 15 day
If blood continues to flow and a woman is still waiting to purify herself, and it exceeds 15 days, then she should fall back to her normal period as a reference. Accordingly, she must make ghusl, and start praying, and in addition make up the prayers for the days which she did not pray since the time when her normal period would have ended.
For example, suppose a woman has a normal period of 6 days. However, during a given month blood continued to flow until 15 days had passed, while she is still waiting for the end of the period. In this case she makes ghusl immediately, and starts praying, and prays all the prayers for the 9 days counted from the end of her normal period until she made ghusl.
The reason for this is that the longest that a woman's menstrual cycle can be is 15 days according to madhab Imam Ahmed and many scholars (al-Mughni 1:388). Thus, if the flow continues past 15 days, then everything after the number of days in a normal cycle is considered istihada (which is blood that flows from a woman due to reasons other than menstruation, such as from a burst vein, hemorrhage, etc.)
Regarding making up the missed prayers, she should pray them consecutively in order by intention, in one session, and she can rest if she gets tired.
Sheikh Muhammed Salih Al-Munajjid
A little bleeding shortly after the period
Praise be to Allaah.This bleeding is not menstruation, so it does not affect the fast, so do not stop praying because of it. But you have to do wudoo’ for each prayer.
Shaykh Ibn ‘Uthaymeen was asked: if a small spot of blood comes out of a woman during the day in Ramadaan, and this (spotting) continues throughout the month of Ramadaan and she fasts, is her fast valid?
He replied:
Yes, her fast is valid, and this spotting does not matter because it is from a vein. It was narrated that ‘Ali ibn Abi Taalib (may Allaah be pleased with him) said: “These spots that are like a nosebleed are not menstruation.”
Sittoon Su’aalan ‘an Ahkaam al-Hayd.
The Standing Committee was asked: Sometimes my period lasts nine days and sometimes ten. When I become pure (i.e., my period ends) and I get up to do housework, it comes back, starting and stopping. If it comes back after the time stated in sharee’ah that a period may last, is it permissible for me to fast, pray and do ‘umrah?
They replied:
The length of the period in your case is the length of time your period usually lasts, which is ten or nine days. When the bleeding stops after nine or ten days, then do ghusl and fast and pray, and do tawaaf for Hajj or ‘umrah, or a voluntary fast, and it is permissible for your husband to have intercourse with you. Any bleeding that resumes after the period is over because of activities that you do or any other reason is not menstrual blood, rather it is due to some sickness or problem, so it does not mean that you cannot pray, fast, do tawaaf or any other acts of worship. Rather you should wash it away, like any other impurity, then do wudoo’ for every prayer, and pray and do tawaaf around the Ka’bah, and read Qur’aan.
Fataawa al-Lajnah al-Daa’imah, 5/426.
Praying when a woman's period lasts longer than normal
Therefore, the woman is not allowed to perform prayer, have intercourse with her husband, etc., until she becomes totally clean , even if her period is longer than usual, unless it exceeds 15 days.
Sheikh Muhammed Salih Al-Munajjid