If the menses lasts longer than usual
Question

                If a woman’s monthly period is usually eight or seven days, then on one or two occasions it lasts longer than that, what is the ruling?.
Praise be to Allah.
    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)