What is the ruling on menstruation continuing throughout the day and causing the omission of prayer?

Menstruation prevents a woman from praying until she becomes pure. Once she becomes pure, she must perform ghusl (definition) (ritual bathing) and pray. It is not permissible for her to pray before purification unless the blood flow exceeds fifteen days, in which case she is considered to be in a state of istihadah (definition) (non-menstrual vaginal bleeding). Scholars have differed regarding her ruling: some say she should rely on discrimination (between menstrual and non-menstrual blood), and some say she should rely on her customary period if she had one previously. There is no blame upon her if she adopts either of these two opinions.