Are the days exceeding the usual menstrual period (7 days) considered istihadah, necessitating prayer and fasting, or does menstruation extend to include all these long days (10 to 18 days) caused by uterine fibroids, thereby prohibiting prayer and fasting?

Days in which bleeding exceeds 15 days: You observe your previous habitual period only, then you perform ghusl (ritual purification bath) and pray; because the maximum period for menses is 15 days, and anything beyond that is istihada (non-menstrual vaginal bleeding).

Days in which bleeding does not exceed 15 days: All of them are considered days of menses. So, when the bleeding stops, perform ghusl and pray.

If you do not know whether the bleeding days will exceed 15 days or not: You consider all of them as days of menses and refrain from prayer and fasting, because bleeding that exceeds your usual habit is considered doubtful menses, and doubtful menses are treated the same as certain menses. Then, if it becomes clear to you that the bleeding exceeded 15 days, then the excess is istihada, and you must make up for the fasting and prayers you missed.