Does the discharge of brown fluid after purification invalidate the purification and necessitate a new ghusl, and what is the specified duration of the menstrual cycle after which the blood is considered istihada, making prayer and fasting permissible?

The habitual period may increase or decrease. Whatever blood a woman sees that is connected to her habit is menstruation, unless it exceeds fifteen days, in which case it is istihadah (non-menstrual vaginal bleeding). Yellowish and brownish discharges before purity are considered menstruation, but after purity, they are not menstruation, though they do invalidate wudu (ablution).