how long to cook sweet potatoes in the oven
1 Bake for 45 minutes for sweet potatoes/yams that are 2-3 inches in diameter. 2  For sweet potatoes that are up to 4 inches in diameter, bake for an hour. 3  For super large sweet potatoes, bake for an hour and 15 minutes. 4  After the time has elapsed, don't open the oven but turn it off. Turn the oven on to 425. 2  Bake for 45 minutes for sweet potatoes/yams that are 2-3 inches in diameter. 3  For sweet potatoes that are up to 4 inches in diameter, bake for an hour. 4  For super large sweet potatoes, bake for an hour and 15 minutes. 5  After the time has elapsed, don't open the oven but turn it off.