If the menses lasts longer than usual

If this woman’s usual period is six days, then it becomes
longer and lasts for nine or ten or eleven days, then she should continue
not praying until she becomes pure. That is because the Prophet (blessings
and peace of Allah be upon him) did not define a set period for
menstruation, and Allah 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),” [al-Baqarah 2:222]. So long as this blood remains, the woman
is in a state of menses until she becomes pure and does ghusl, then prays.
If in the following month her period is shorter than that, then she should
do ghusl when she becomes pure, even if it is not as long as it was in the
previous month. What matters is that when the woman has her menses she
should not pray, whether the menses is in accordance with what was
previously the norm or it is longer or shorter. And when she becomes pure
she should pray. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him)