Depends on the oil you use. I prefer K&N filter oil, you can get a can of it at any auto parts store and one can has been lasting me about a season and a half. Avoid the sticky air filter oils, they do not trap dirt as well as they should. Also, don't use cheap filter foams, they tend to be more porous and let more dirt through. You should also be using a prefilter. I take the main filter and oil it up heavily in a ziploc bag and work the oil into it. Then I put it inside the prefilter and squeeze any excess oil into the prefilter.