It's recommended that cooking oil in the deep fryer can be used up to 8 times before it needs replaced but that is mostly for food establishments.
At home the cooking oil can last a lot longer and you should get to deep fry at least 100 times with the cooking oil in the deep fryer.
The oil used in your deep fryer can last up to 3 months and sometimes longer depending on how often you deep fry foods.
I usually change out my deep fryer cooking oil every 3 months but sometimes I'll wait until 4 months.
You can keep the deep fryer oil until it starts to smell sour or turns a different color and if the cooking oil has a bad smell to it then dispose of the cooking oil and put fresh cooking oil into your deep fryer.
As long as you're replacing the cooking oil every 2 to 3 months the cooking oil will usually not have enough time to go bad.
I keep a lid over my deep fryer after it's cooled down to keep the cooking oil from going bad as soon.