While fresh corn on the cob can be left out for a bit, you'll want to make sure you don't let it sit out overnight or all day. Corn left out too long is likely to develop mold and other signs of spoilage, especially when not refrigerated. If your corn has been sitting at room temperature for more than a few hours, discard it rather than risk getting sick from moldy food. However, cooked corns may last at little as two hours.