How long can double vision last for after concussion?

Variable duration. Dependent on myriad variables such as severity concussion/contusion, damage to one or more muscles or nerves supplying the muscles that move the eyes or swelling/edema in eye or orbit, or fractures in the bony orbits around the eyes which may restrict normsl eye movements after trauma to the head and orbits.
Double vision. Can be as long as months depending on whether there is damage to any of the cranial nerves that affect eye movement or areas of the brain that coordinate eye movement.