If the Menstrual Period continues for more than the Normal Number of Days
Question :
If a woman's normal monthly period is seven or eight days, then it continues once or twice for longer than that, what is the ruling?
Answer:
If the normal period of this woman is six or seven days, then it becomes longer, lasting for eight, nine, ten or eleven days, then she remains (in menstruation) and she should not pray until she becomes clean. This is because the Prophet did not define any particular number of days for the menstrual cycle. And Allah, the Most High says:
They ask you concerning menstruation. Say that it is an Adha (a harmful thing for a husband to have sexual intercourse with his wife while she is having her mense)
So when this blood continues, the woman continues in that state, until she becomes clean and performs Ghusl and prays. And if, in the following month, her period is shorter than that, then she should make Ghusl when she becomes clean, even if it was not of the same number of days as the previous period. The important point is, that when a woman has her period, she does not pray, whether the period is the same length as the last one, or it is more or less when she becomes clean, she must pray.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 130-121