What is the maximum duration of postpartum bleeding (nifas) after which a woman is permitted to pray?

The maximum duration of postpartum bleeding (nifas) is forty days, according to the more preponderant opinion. Anything beyond that, if it is continuous with the nifas blood, then it is irregular bleeding (istihada), unless it falls within the time of your regular menstrual period or has the characteristics of menstrual blood known to you. If it is not continuous, meaning you became pure from nifas and then new blood appeared, then it is menstruation, even if your custom is for menstruation to be delayed for a long period after childbirth.