If you are like me, and forgot when you last changed the air filter, independent of any scheduled maintenance of course, simply use a permanent marker and write down the month/year on the upper plastic border of the air filter. So when you take off the cowl, you will see the date.

 

i do this on all my vehicles but also record the last oil change on the air filter. i also log it in the back of the owner's manual. figure it's safer to have the info in two places for me.