This varies depending on the volume of food being fried and the type of food. If large amounts of food are fried oil seems to last forever. This is actually deceiving, as the fryers have to be topped off frequently in a situation like this, which dilutes the old oil with new and makes it appear that it lasts a long time. In normal-volume restaurants four or five days is normal for general purpose frying in lightly hydrogenated vegetable oil, assuming the oil is filtered two to three times a day. In applications that have freshly-breaded product, with a large amount of sediment associated with it, the oil may last only a day.
Examples are the onion loaf, or the onion cut to resemble an opening flower.
These products throw off so much flour that most people dedicate one fryer to cooking only this product and plan on disposing of the oil at the end of the night.