If a person was in a car accident and has badly damaged kidneys, how long does it take to heal? he is on dialysis....

Depends on injury: The prognosis for acute kidney failure depends on type of kidney injury. I would expect that with car accident, cause of kidney failure would be massive muscle damage that led to toxic effect on kidney from proteins released from muscle. Often improves over days or weeks, but unfortunately is sometimes permanent. Ask nephrologist what the expected outcome is. Hope things improve soon.

It depends: If the kidneys were directly injured by the trauma it can be a long to permanent injury. If the kidney injury is from the shock and low blood pressure from other traumatic injuries they can recover more quickly. Your doctors who know the patient directly could give you a better answer than we can.