If the bleeding lasts for more than fifteen days, it is clear that it is irregular bleeding (istihaadah) and she should make up the prayers of any days beyond the duration of her usual period
Question

Last Ramadan, my wife’s period was delayed by approximately three months, then it finally came on 16
th Sha‘baan, but it was not as it had been before, as it was very light. On 29th Sha‘baan, in the same month, it became heavy. The doctor prescribed some medicine for her, but it did not stop. She went to another doctor on 26th Ramadaan, and she prescribed another kind of medicine for three days, then the period stopped. But my question is: does she have to make up 28 days, because she counted seven days at the beginning of Ramadan, and did not fast during that time, then she completed the fast for the rest of the month, assuming that it was istihaadah (irregular bleeding). Is her fasting valid or not, or does she have to make up all 28 days?
She only fasted 15 days, based on the fatwa mentioned, but I don’t know whether her actions were valid or if she sinned because she did not fast 28 days.
Praise be to Allah.Firstly:
The basic principle is that the bleeding that occurs is menstrual blood, regardless of whether there were 15 days between it and the previous bleeding, or 13 days, or less than that. If the bleeding continues beyond fifteen days, then the extra is istihaadah (irregular bleeding), so the woman should do ghusl and fast and pray. Then in the following month she should stop praying and fasting for the duration of her usual period, then do ghusl. If she did not have a regular cycle before that, then she should distinguish between types of bleeding and act accordingly. If she cannot distinguish between them, she should stop praying and fasting for the maximum duration of menses.
Please see the answer to question no. 68818 .
Your wife made a mistake by stopping praying for only seven days at the beginning of Ramadan, because it would have been valid to do that only if it was established that this was istihaadah, and that was not established at that time. It is well known that the length of a woman’s period may increase or decrease, so perhaps her period had increased to ten days or fifteen days.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: My monthly period came and is still with me; it has gone on for twelve days until now. This woman’s usual period lasts for only seven days, so she did not pray during the first seven days, then she did ghusl and prayed, based on her usual cycle. Is what she did correct? Can she fast or not? Is she permissible for her husband during the remaining days? Please advise us, may Allah bless you.
He replied: What she did would be correct if the bleeding had continued beyond fifteen days. But before fifteen days, it is possible that the length of the monthly period had increased, because the duration of women’s periods sometimes increases and sometimes decreases.
So we say to this woman: Wait until it reaches fifteen days, then when it reaches fifteen days, do ghusl and pray. Then the following month, stop praying for the duration of your usual period, because a woman cannot be regarded as suffering istihaadah until the bleeding has passed fifteen days. That is because if the bleeding continues for more than fifteen days, then in that case, she will be bleeding most of the time. In that case she should refer to her usual cycle as it was before this sickness came to her. She is not permissible for her husband until fifteen days have passed, at which time she should do ghusl, then she becomes permissible for him. Then in the following month, she should stop praying for the duration of her usual period only, then she should do ghusl and pray, and then she becomes permissible for her husband." (Al-Liqa’ ash-Shahri  69/20).
Secondly:
If it becomes clear that the woman is suffering from istihaadah, because her bleeding lasts beyond fifteen days, and she fasted on the days after the days of her usual period ended, then her fasting is valid, because it became clear that the bleeding was istihaadah and not menses. However, she made a mistake as we have noted, because she should have refrained from fasting for fifteen days, because at that time she did not know that it was istihaadah.
Shaykh Ibn Baaz (may Allah have mercy on him) said: If the bleeding stops on the eighth day, on the ninth, on the tenth, or the twelfth, meaning that it went beyond the usual duration, but it stopped, then this is called menses according to the correct view, because the duration of the period may increase or decrease, and the bleeding may be continuous or interrupted. For example, one day she may see bleeding and one day she may see that she is not bleeding at all; this may be described as mixed. She should do ghusl [and pray] on the day when she is not bleeding, and she should refrain from praying on the day when she is bleeding, and her period may be described as mixed. Then when the total reaches fifteen days, or fourteen days, then the days on which there is bleeding are menses, and the days on which there is no bleeding are not menses. Then if that goes on for longer than fifteen days, it is to be regarded as istihaadah, as was stated by the majority of scholars (may Allah have mercy on them).
This is the correct view: the maximum duration of menses is fifteen days. Then if it goes on for longer than that, it becomes istihaadah, so she should pray and fast, and do wudoo’ after the time for each prayer has begun, and she should make up the prayers of the days on which she did not pray after the end of her usual period, up to the fifteenth day. If she fasted during these days, then her fast is valid on the days after the end of her usual period, because it became clear that that was istihaadah and not menses." (Fataawa Noor ‘ala ad-Darb 5/401).
And Allah knows best.