Are the days exceeding the first seven days considered istiḥāḍah (irregular bleeding), and what is the ruling on prayer at that time, if the menstrual cycle lasts 15 days, with 7 days of normal bleeding and 8 days of dark brown and scant bleeding, and the period of purity is only 10 days?

The minimum period of purity (Tuhr) between two menstrual cycles is thirteen days according to the Hanbalis, and fifteen days according to the majority of scholars (Jumhur (definition)). If the duration of purity is less than thirteen days, then the total number of days cannot be considered menstruation, and the woman is considered to be experiencing irregular bleeding (Istihadah (definition)). She must refer to her previous menstrual habit, perform Ghusl (definition) (ritual bath) after it, and consider any days beyond that as Istihadah.