How can the time for ritual bathing (ghusl) after menstruation be determined, given the change in the number of days of bleeding, where the usual custom was 7 days plus 3 days of "istizhhar" (waiting to ensure purity), making a total of 10 days?

What the questioner is doing is wrong. The correct approach, according to the Maliki school, is that if a woman with a regular menstrual cycle experiences an irregularity in its duration, she should consider her usual period. If the bleeding does not stop, she should extend it by three days. Then, she should perform ghusl (definition) (ritual bath) and pray, and the bleeding after that becomes istihada (non-menstrual bleeding).

If her usual period is seven days, she should extend it by three to make it ten days. These ten days will be considered her new regular period for the next cycle. If the bleeding does not stop after ten days, she should extend it by three more days, making the period thirteen days, and so on, until it reaches fifteen days, which is the maximum duration for menstruation, and there is no extension beyond that.

Furthermore, a regular period is established by a single occurrence, and the scarcity or abundance of blood is not considered in distinguishing purity from menstruation.