It takes 30-40 minutes to bake sweet potatoes in an electric oven. Sweet potatoes are first washed and dried, turn the oven on to 150 degrees to preheat for 5 minutes, then lay a layer of tinfoil in the oven, turn the oven on to about 200 degrees, put it in and bake for 20 minutes, turn it over and bake for another 10-20 minutes.