A vehicles air filter usually needs changed every 1 or 3 years depending on how often you drive your vehicle.
After a year the air filter usually begins to get dirty especially if you drive in dusty conditions or drive more often.
Air filters though are usually good for 15,000 to 30,000 miles however when you drive on unpaved roads or in dusty conditions changing the air filter more often is a good idea.
I change my air filter every 1 or 2 years which is when I get around 15,000 miles on my vehicle and air filters are pretty cheap and easy to replace.
Replacing the air filter will also improve your gas mileage because the vehicle will run better and get more air to the engine like it should.
A bad or dirty air filter can cause your engine to Misfire, use more fuel, reduce engine horsepower and even cause your vehicles check engine light to come on.