When should a woman begin to pray if her menstruation continues for more than ten days, knowing that her period began on Monday evening, and is Monday counted as a full day?

The maximum duration of menstruation is fifteen days. As long as the customary period does not exceed this number, one must refrain from prayer and all other prohibitions related to menstruation. If the customary period is twelve days, it is not permissible to perform ghusl (definition) (ritual purification bath) before its end as long as menstruation continues. The questioner's statement that prayer becomes obligatory after the tenth day is incorrect, as long as the customary period is longer than that and menstruation has continued. If the bleeding stops and she sees purity (dryness or white discharge) before the end of her customary days, she is considered pure. Day or night bears no significance in purity; whenever purity occurs, she should pray. If menstruation continues for more than fifteen days, the blood is considered irregular (istihada) and she must perform ghusl and pray. As for counting the day, if the blood appears after sunset, then the morning of that night counts as part of the days of menstruation. If it appears before sunset, then the day on which menstruation began counts as part of the days of menstruation.