All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
When a woman's menses stops, it is not permissible for her to delay having a ritual bath until the time of the prayer is over. When her menses stops, it is an obligation on her to perform the prayer, yet for the prayer to be valid, it is a condition for her to have a ritual bath and purify herself from the state of impurity. So, it is an obligation on her to have a ritual bath in order to become pure from menses. It is not permissible for her at all to delay her ritual bath until the prescribed fixed time of the prayer is over. Moreover, she should not wait for the odd days to purify herself, as no scholar, has ever mentioned this nor was it mentioned in the Book of Allaah or the Sunnah of the Prophet .
Therefore, it is an obligation on her to have a ritual bath before the end of the prescribed time of the prayer, as she should offer the prayer before then. Also, she should not allow her husband to have sexual intercourse with her before purifying herself by having a ritual bath.
Allaah Knows best.