Her period stopped for six months then it came back and she has been bleeding for more than three weeks
The basic principle is that the blood that comes from the woman is menstrual blood, so long as it does not last the entire month or only stops for one or two days (in the month), in which case it is istihaadah (irregular, non-menstrual bleeding). This is the correct scholarly view, so there is no maximum limit for the menses.
The menses may increase or decrease, and it may come earlier or later, or it may stop for several months.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Some women may remain without menses for four months, then get the menses for an entire month, as if – and Allah knows best – it is withheld and then all comes at once. And some women may menstruate for three or four or five or ten days every month.
End quote from ash-Sharh al-Mumti‘, 1/402
For more information please see question no. 131541
It is well known that the woman who is menstruating should not pray or fast. As for the woman who is suffering istihaadah, she should pray and fast, and her husband may have intercourse with her.
Based on that:
What is happening to you now is menses, so long as it does not last a complete month or one or two days less than that, in which case it is istihaadah; and take this into account for the following month.
See also question no. 68818 on what to do in the case of istihaadah.
And Allah knows best.