According to Muslim scholars, post-natal bleeding has no minimum duration, while its maximum duration is 40 days.

In his well-known book, Fiqh As-Sunnah, the late Sheikh Sayyed Sabiq states:
Nifas (post-natal bleeding) has no minimum duration, for it can stop right after the birth. (There is a scholarly consensus that) the maximum duration is 40 days. Umm Salamah narrated: “During the lifetime of the Prophet (peace and blessings be upon him) the post-childbirth woman would be in confinement for forty days.” (Reported by Ahmad, Abu Dawud, At-Tirmidhi, and Ibn Majah)
At-Tirmidhi states: The knowledgeable Companions of the Prophet (peace and blessings be upon him), the following generation (tabi`un) and those who came later agree that a woman experiencing post-natal bleeding must stop praying for 40 days unless the bleeding stops, in which she must perform ghusl (bath of purification) and start praying. If she sees blood after 40 days, most scholars say that she is not to stop praying.”