Is it correct to not perform ghusl until after the fifteenth day from the onset of menstruation, due to the continued discharge of a viscous yellow fluid after the cessation of blood?

If the bleeding stops and the woman sees purity, then any yellowish or brownish discharge after that is not considered menstruation. However, if the yellowish discharge is connected to the blood, it is menstruation until it stops. Once it stops, and she sees dryness or her usual white discharge, she is considered pure. If the blood and any connected yellowish discharge continue for more than fifteen days, it is considered istihada (non-menstrual bleeding). In this case, she must refer to her previous habit if she knows it. Otherwise, she should differentiate, considering the days she sees red blood as menstruation, and anything else as istihada.