Depends on what you are frying, if you fry a lot of potatoes that will help with keeping the oil cleaner that if you fry nothing but meats. Generally speaking, your nose will let you know when it is time to change the oil.
How Long Can I Keep Oil In A Deep Fryer?
Oil can last smelling okay for a while, but after frying fish and other meats, pretty gross and can cause a mean stomach flu