I say, and with God's success: If the blood exceeds her usual period and goes beyond ten days, then she returns to her usual period, and what she sees during her usual days is considered valid blood as if it had not exceeded ten days, and the excess beyond her usual period is considered istihada, which is invalid blood. However, if it does not exceed ten days, it is counted as menstruation, which is a new usual period for her. See: Manhal al-Waridin, p. 35, and others. And God knows best.