The frequency of changing fryer oil can vary depending on the volume of food being cooked, the type of food, and the type of oil being used. As a general guideline, oil in deep fryers should be changed after every 5 to 7 days of moderate usage. However, in a high-volume cooking environment, it's recommended to assess the oil quality daily and change it when it shows signs of degradation, such as off odors, flavors, or excessive smoke at lower temperatures.