If a menstrual period lasts longer than usual
Question

                If the monthly period comes and lasts longer than usual like bleeding, is it permissible for a woman to pray and fast in this situation?
Praise be to Allah.
      If the period lasts longer than usual, the correct thing is 
      to follow what is usual. If her period is usually five days, but this time 
      it is six or seven or eight, the right thing to do is to refrain (from 
      praying etc) so long as the blood is flowing, and regard it as menses, and 
      not pray or fast, and it is not permissible for her husband to have 
      intercourse with her in this case, because the period may be longer or 
      shorter; this is normal among women. But if she bleeds for longer, she 
      should not pray and fast, and if it is shorter and she sees that the 
      period has ended, she should do ghusl and she becomes permissible for her 
      husband, and she should pray and fast. This is the correct response in 
      this case. 

    But if she becomes pure and does ghusl, then she sees 
    yellowish or brownish discharge, this yellowish or brownish discharge is not 
    regarded as anything (i.e., is not menses), so she should pray and fast, as 
    Umm ‘Atiyyah (may Allah be pleased with her) said: We used not to regard the 
    brownish or yellowish discharge that comes after tuhr (becoming pure) as 
    being of any significance. 

    If she does ghusl after her period, then sees yellowish or 
    brownish discharge, this does not count as anything or prevent her from 
    praying or fasting or having intercourse with her husband. But in the case 
    of pure blood that is clearly blood, then she should stop praying etc., even 
    is it was interrupted. Whether it is continuous or interrupted, she should 
    stop praying etc. according to her regular cycle. End quote. 

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