When is the time for purification from menstruation if its duration exceeds ten days, and it normally does not exceed one week?

What is mentioned in the question is the change in the custom (period duration). The soundest opinion is that an increase in menstruation is considered menstruation from the first occurrence, as long as it does not exceed fifteen days. If it exceeds that, then the woman reverts to her usual custom, and the excess is considered istihada (non-menstrual vaginal bleeding). She must then make up for the prayers and fasts she missed.