What is the ruling concerning blood that flows after seven days from the onset of menstruation, knowing that its natural duration previously was seven or eight days, and now it continues for up to nineteen days with intermittent brown discharge? And is it considered to be istiḥāḍah based on the doctor's advice?

The maximum duration of menstruation is fifteen days, according to the majority of scholars. If the discharge of blood, or any connected yellowish or brownish discharge, exceeds this period, then the woman is considered to be in a state of istihadah (non-menstrual bleeding). She should refer to her previous habit, observe that period, and perform ghusl after its completion. Whatever exceeds that is considered istihadah, for which she should perform ablution for each prayer after its time has begun. She is then subject to all the rulings applicable to ritually pure women.