Sometimes it is difficult to determine when your car needs maintenance, especially when it comes to items like air filters - in other words, things you don't think about... ever.
To determine whether it's time for a switch, think about where and how you drive your car. According to Yahoo! Autos, there's a big difference between driving 20,000 miles on a freeway and spending a month on rural dirt roads.
Everything depends, essentially, on how dirty your car gets. If it's currently covered with crud around the tire spaces, it might be time to replace your filter.
Of course, the filter should be switched out before it greatly blocks air flow, but surprisingly, it's reportedly good to have a slightly dirty one. With a little bit of road waste, the tiny particles get blocked as well.
Trucks tend to have an air filter monitor that tracks its progress, but if not, the best you can do is guess, states Yahoo!
If you don't know this already, when air flow is significantly blocked, fuel economy and performance begin to suck wind until the filter gets replaced, so be sure to check it every once in a while.