Cooking oil can be used for 6 months to a year as long as it's been kept covered when not in use.
However it's best to change the cooking oil every 3 months to 6 months to keep the cooking oil fresh enough and for the cooking oil to cook the food fast enough and properly.
After 6 months the cooking oil loses a lot of its virtues and is still usable however it doesn't cook the food as good as fresher cooking oil does.
I change my cooking oil every 3 months however I sometimes let the cooking oil go for 6 months if I'm running short on money at the time.
Cooking oil that is older will not do you any harm although the food won't taste as good as it would when cooked with fresher cooking oil.