What is the ruling if the length of the menstrual period is increased because of taking the contraceptive pill?
Question

                If a woman takes the contraceptive pill and her monthly period changes from five days to ten days, is it permissible for her to stop praying during the additional days?.
Praise be to Allah.
      Yes, the monthly period may alter and change from five days 
      to eight or ten or seven, and there is nothing wrong with that. If the 
      period changes like this, the woman should not pray or fast. The period 
      may also change and become shorter; it may be seven days then the bleeding 
      stops in five days. Or it may be seven days but then continue until ten 
      days. So she should not pray when there is bleeding, up to fifteen days. 
      If it reaches fifteen days, the majority of scholars say that it becomes 
      istihaadah (irregular, non-menstrual bleeding) after that, so she should 
      go back to reckoning her usual period, such as six or seven days, and the 
      rest is istihaadah, She should pray and fast at that time, doing wudoo’ 
      for each prayer, washing herself with water and using a piece of cloth or 
      pad. She should continue doing that until her regular cycle resumes. End 
      quote. 

    Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)