To grill potatoes in aluminum foil it takes a minimum of 30 minutes for the potatoes to fully cook on the grill.
Cook the potatoes on the grill at least 30 minutes and be sure to turn the potatoes over at least once during that time while cooking them on the grill.
The potatoes should be fully cooked within those 30 minutes but if not you may have to cook them another 4 to 5 minutes.