The scenario that can occur with menstruating women: A woman who typically has an irregular cycle, and after 7 days, the type of blood being discharged now is different from the hayth blood. Different color, consistency, etc. Does she need to wait until the maximum days are met, or can she resume praying?

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If she feels that after those 7 days her period has changed to istihatha, she can start praying.
If she is not able to distinguish between the hayth and istihatha (it goes back and forth), then she should wait until she reaches the maximum length.