You can leave the oil in the fryer as long as you store it in a cool, dark location. How long it will be good is a function of a number of things such as, what was fried in it - high protein items such as meats deteriorate oil faster than veggies, whether or not the oil was strained after each use to remove frying residue, if the oil was heated up to or past its smoke point and the type of oil used. Oil will darken with reuse. If foods darken too quickly (before they are cooked), it's a sign the oil needs to be changed. With each use, the oil's smoke point drops, resulting in the early darkening.