How long is the pus drainage from a ruptured ear drum supposed to last?

Usually a few days: The drainage from a middle ear infection that causes a ruptured ear drum will typically stop in 4 or 5 days. These perforations are generally quite small and many heal within a couple of weeks. Ongoing drainage may indicate a problem beyond a straight forward middle ear infection. Infections can spread into the mastoid or air cell tracts in front of the inner ear. See a doctor.