When baking an unpeeled potato wrapped in heavy-duty aluminum foil directly on the rack, it typically takes about 60 to 75 minutes at 425 degrees Fahrenheit (218 degrees Celsius) for the potato to become fully cooked and tender. The exact baking time can vary depending on the size of the potato. To ensure it is fully cooked, you can check for doneness by piercing the potato with a fork or knife. If it easily slides through the potato without resistance, it is cooked and ready to be enjoyed.