It's important to change dirty air filters. Compare the clean white filter with a filthy one that needs to be replaced. Check the air filter. If you can see deposits of dirt and grime on the air filer, it's time to clean or replace it. These days, most mower engines use disposable paper filters.
Conditions determine how often a filter needs to be replaced.
But in wet conditions you might be able to go several years without replacing it. Cleaning the underside of the mower deck with a putty knife can make a real difference in cutting and clipping dispersal. Plugs just don't foul as much as they used to. It may not even be necessary to change the plug every spring, but it can't hurt either. Today's Homeowner, a division of Times Mirror Magazines.