If the bleeding lasts for more than 15 days is it still regarded as menstruation, and can she fast?
Question

                I jst want to know that i have menhorrehia and my periods are irregular if i go over 10 or 15 days am i allowed to pray or not?.
Praise be to Allah.
      There is no definitive evidence in sharee’ah to prove the 
      minimum or maximum length of menses. Shaykh Ibn ‘Uthaymeen was asked: Is 
      there a definitive number of days for the minimum or maximum length of 
      menses? 

    He replied: 

    There is no definitive number of days for the minimum or 
    maximum length of menses, because Allaah 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), therefore, 
    keep away from women during menses and go not unto them till they are 
    purified (from menses and have taken a bath)”

    [al-Baqarah 2:222]
So Allaah did not make 
    the period of abstention a certain number of days, rather He stated that the 
    period of abstention ends when the woman becomes pure. This indicates that 
    the reason for the ruling is menstruation, and whether it is present or not. 
    When menstruation is present, the ruling applies, and when the period ends 
    the attendant rulings no longer apply. Moreover there is no evidence for 
    putting a time limit on menses, even though there was a need (at the time of 
    the Prophet  (peace and blessings of Allaah be upon him)) to explain the 
    definition if any such definition had existed. If there was such a 
    definition, it would have been explained in the Book of Allaah or the Sunnah 
    of His Messenger  (peace and blessings of Allaah be upon him). Based on 
    this, every time a woman sees the kind of blood which is known to women as 
    being menstrual blood, then it is menstrual blood without that being 
    restricted to a certain time, unless the bleeding is continuous and never 
    stops, or it stops only for a short time such as one or two days in the 
    month, in which case it is istihaadah (non-menstrual vaginal bleeding). 

    Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 
    11/271 
Based on this, it is not 
    permissible for you to pray except after the period has ended and you have 
    done ghusl (full ablution). The ending of menstruation may be known from one 
    of two signs: either the emission of a white discharge which comes at the 
    end of the period, or by total cessation of bleeding. 

    For more information, see the answer to question no. 
    5595, 
    where the matter is discussed in more detail. 

    And Allaah knows best.