What is the ruling regarding blood that continues for more than three weeks after the cessation of the menstrual period, the nature of which is sometimes heavy and sometimes light, and does this blood prevent one from praying?

If menstruation is delayed for several months and then occurs, lasting for more than three weeks, then the menstrual blood is only the first fifteen days. Anything beyond that is irregular bleeding (istihada). Ghusl (ritual purification bath) is obligatory after the end of the menstruation period (15 days), and the woman is then considered pure, so she may pray and do everything that pure women do.