every 3000 miles or 3 months?, only kidding. i use fresh oil every time i use my fry daddy, but i only use it once in a blue moon, and i haven't made all that much in it, so i'm not sure just how much you can cook in it, or how long the oil will be good for.
a chinese restaurant by my job makes an excellent deep fried 1/2 chicken, but you have to catch them early on the day they change the oil. they partially fry a big load of chickens, and put most to the side to be re-fried later on when someone orders one. if you get there early, they will fry one all the way until done in one shot. they taste so much better this way. after a few days tho, the oil has been adulterated by so much chicken fat that the food fried in it doesn't taste very good.
fresh oil, at the right temp, and for the appropriate amount of frying time is the way to go...