If this woman’s usual period is six days, then it becomes longer and lasts for nine or ten or eleven days, then she should continue not praying until she becomes pure. That is because the Prophet (blessings and peace of Allah be upon him) did not define a set period for menstruation, and Allah says (interpretation of the meaning): “They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have a sexual intercourse with his wife while she is having her menses),” [al-Baqarah 2:222]. So long as this blood remains, the woman is in a state of menses until she becomes pure and does ghusl, then prays. If in the following month her period is shorter than that, then she should do ghusl when she becomes pure, even if it is not as long as it was in the previous month. What matters is that when the woman has her menses she should not pray, whether the menses is in accordance with what was previously the norm or it is longer or shorter. And when she becomes pure she should pray. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him)