Cooking potatoes in foil on the grill takes around 30 to 40 minutes depending on how well cooked you like the potatoes.
I cook my potatoes on the grill for 30 minutes which is good enough for me.
After you place the potatoes in the foil and add any butter or any other ingredients that you want to add to the potatoes then place the potatoes in the foil onto the grill after getting the grill heated up.
Then let the potatoes grill for 15 minutes then after the 15 minutes turn the potatoes in the foil over and let the other side of the potatoes grill.
Then after 30 minutes of grilling the potatoes should be done and ready to serve and eat once they cool down.