The time to bake sweet potatoes in the oven is determined by the size of the sweet potato, usually 20 minutes. Adjust the heat to 220 degrees, it will take half an hour to 40 minutes, which is for medium-sized sweet potatoes. Increase or decrease the time depending on the size of the sweet potato. After half an hour, remove the sweet potatoes and turn them over to allow them to heat evenly. Re-heat for 20 minutes to half an hour, at which point the sweet potatoes will be fully cooked.