If the menses continue for more than their normal length

Question: If a woman normally has menses for eight or seven days but once or twice she has them for a longer period, what is the ruling concerning that?

Response: If that woman normally has her menses for six or seven days and then they become longer, becoming eight, nine, ten or eleven days, then she remains not praying until she becomes pure. This is because the Prophet (sal-Allaahu ‘alayhe wa sallam) did not set any specific limit for menstruation. Allaah says in the Qur.aan, "They ask you concerning menstruation. Say: It is a harmful thing..." (al-Baqarah 222). As long as that blood is flowing, the woman remains in her state of menses until she becomes pure [the blood stops] and she makes ghusl and prays. If, in the following month, the blood comes for a shorter period of time, she makes ghusl when the blood stops even if it was not as long as the previous period. The important point is that as long as the woman is having menses, she remains in that state as long as she has bleeding and she does not pray, regardless of whether that amount of time is the same, longer or shorter than her previous menses. When the blood stops, she prays.

Shaykh Ibn 'Uthaymeen
Fataawa al-Mar.ah