When does a woman with irregular bleeding (istihada) perform ghusl to end menstruation: Is it after 15 days (on the 16th day) or on the 15th day?

The blood a woman sees could be menstrual blood or blood from a non-menstrual discharge (istihadah). If a woman menstruates for fifteen days, she should consider the time she first saw the blood, and at the corresponding time fifteen days later, she performs ghusl (ritual bath). So, if she saw menstruation at noon on the first day of the month, she performs ghusl at noon on the sixteenth day.