How many days after a hairline nasal fracture will there be mild dull facial or head pain?

Depends: Depends on the extent of the injury. Even though there was only a hairline fracture of the nose, its still a broken bone. And there might have been injury to the brain in the form of a concussion. All these things can cause pain after the injury. If the pain has not subsided within a week I would consult a physician for further evaluation