It depends on the doctor. Follow your doctor's recommendations. The conservative way is to wait until all the hairs grow out in about a year. Other case by case exception is if there is a single spot that has been obviously overlooked, you can do it any time. But this is ultimately your doctor's decision to make.
You should wait at least one year before having any touch up or revision done to the area you had the grafts placed even if you feel the areas of concern were missed or overlooked. Follicles can be placed in areas where there is existing fine hair. Your scar would need to be evaluated to determine if it is suitable for follicles to be implanted (good blood supply, etc.)