The majority of scholars say that menstruation shouldn’t exceed 15 days a month. This means that on the 15th day, you should shower and pray. The blood coming out not is called Istihada.

As long as you know that date that your period comes in, you should stop praying in these days and then perform Ghusul and you can pray until the next month.

What if a woman is unsure of the blood she sees,if its is menstural or something else (blood-vessel bleeding- not sure) ?

Menstruation and post-natal bleeding | March 7, 2014 4:52 am
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I had an abortion of 11 weeks. Now its my 20th day after that and my bleeding has been stopped after 12th day. So am I pure now or I have to wait for Salah for more 40 days?
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I have a normal menstrual bleeding but will not stop at 5 days until after 2 weeks or take a drug to stop it. Should I take my bath and pray after 15 days even though the blood has the same characteristics and smell as the menstrual blood?