Is an increase in the duration of menstruation beyond fifteen days considered istihadah (non-menstrual vaginal bleeding), even if the blood has the characteristics of menses and the flow is slow?

The maximum duration of menstruation is fifteen days according to the majority of scholars, and anything beyond that is considered irregular bleeding (istihadah). The Malikis—whose madhhab is prevalent in the questioner's country—hold that a woman with a regular cycle observes her usual menstrual days. If the bleeding continues, she waits three more days (for istizhar - a precautionary period), provided that the total number of days does not exceed fifteen. If the bleeding does not stop after that, she performs ghusl and prays.