When a woman’s menses has finished, does she have to perform ghusl immediately or can she wait a while as long as the prayer time is not over? For example, if asr is at 5.15 pm and Maghreb is at 9.15pm and her menses ends at 5.30pm can she perform ghusl at 6.15pm?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

There is nothing wrong in delaying Ghusl (ritual bath) after becoming pure from menses as long as it does not lead to missing the prescribed time of the Prayer. However, it is not permissible to delay the Ghusl if by delaying it one will miss the prescribed time of the Prayer. This is because praying on the prescribed time is one of the greatest religious obligations which Allah Almighty warned us not to neglect it. Allah Says (what means): {But there came after them successors who neglected prayer...} [Quran 19:59]

To sum up, it is obligatory for a woman who becomes pure from menses, during the time of any prayer, to hasten to perform Ghusl before the time of that prayer ends and then perform the Prayer on time.

Allah Knows best.