How long does it take for skin to grow back after a leg road rash?

3 weeks. These are essentially partial thickness burns and can be treated with washing with soap and water 2-3 times/day, application of Bacitracin ointment and a dry dressing. These types of injury should be healed in 3 weeks--if they are not, or if they do not seems to be improving normally, you should see a physician to see if there is an additional problem.
Depends. By "road rash", i assume you mean severe abrasions of the leg secondary to falling off of a motorcycle or similar vehicle and sliding along the pavement. Healing time depends entirely on how extensive and deep the abrasions are, how quickly they were attended to, whether bits of road are still in there, and whether an infection develops. Unless they are very superficial figure a month, minimum.