The type of foods cooked in a deep fryer, as well as its frequency of use, will determine how often you will need to change the oil. When the oil starts to smell or the quality of the food cooked in that oil starts to degrade---changes colour or tastes funny---it is time to change it out. If the oil foams or smokes when you use your deep fryer, it is well past its expiration.
The most accurate way to test the quality of deep fryer oil is via chemical tests. Testing devices and test strips that measure the oil's total polar compounds (TPC) or free fatty acids (FFA) are available for restaurant owners and other people in the food industry. Both tests will inform you the moment the oil is past its prime.
Although chemical tests provide the most accurate timeline for changing deep fryer oil, they can be somewhat expensive for the home cook, and are used primarily by the food industry. Unless you own a restaurant, the most practical way to test for oil quality is by simple observation. If the oil smells or the food starts to change in colour or taste, replace it.