Mostly, menses last (15) days, and more than that is Istihadah (bleeding outside the time of monthly period). Thus, a woman should perform Ghusl (ritual bath) to purify herself from menses, pray, and make ablution for every prayer when its time is due. She should also consider her regular menses in terms of time and duration. So, if she knows her regular periods, then she should count the days of bleeding during this period as menses, and after those days, she is considered as having Istihaadhah. For example, if her period extends from the 20th till the 27th of every month, then this is her menses, and whatever before or after that is Istihadah. Therefore, she is prevented from prayer and fast during her menses, but when it ends , she is at liberty to perform Ghusl and pray.