How does a woman experiencing prolonged bleeding (Mustahadah) calculate the eight days of her monthly cycle, and when is the first day considered to begin if the bleeding starts a little before sunset?

A woman experiencing irregular bleeding (istihadah) should refer to her previous regular menstrual cycle. If her cycle was eight days, she should perform ghusl (ritual bath) after the eight days have passed. The day is calculated from the time the blood is first observed. So, if the blood appeared at noon, the day ends at noon the following day. The eight days would thus conclude at noon after those days have elapsed. Therefore, in the scenario mentioned, the questioner should perform ghusl before the Maghrib prayer on the eighth day.