Should I change sparkplugs/air filter based on months or miles?

The recommended maintenance schedule says it's time to change my sparkplugs now since it's been so many months, but not yet by miles. Should I change them now or wait until I hit the recommended mileage? I know it's %26quot;which ever comes first,%26quot; and that makes sense to me for things like coolant and belts which will degrade over time no matter if the car is driven or not. But for things like sparkplugs and air filters?
Should I change sparkplugs/air filter based on months or miles?
Spark plugs,air filter,wipers are things that should be changed by mileage or wear. Mileage and time are just guides to let the average user know these things should be inspected for wear. Look at the spark plug electrodes, are they rounded or worn to one side? or are they stil relativley square shaped. (some platinum plugs are round shaped) look for uniformity. Filters are changed when they are dirty, blocking air, they also get wavey looking if they've gotten wet.

This is a sign that they need replacement. Depending how and where you drive has a great impact on how often filters are changed.
Should I change sparkplugs/air filter based on months or miles?
Miles.

How will they wear out just sitting there?
They recommend months or miles because it's much easier than measuring what truly matters...hours in use.



Fact of the matter is, these are recommendations, not requirements.



A requirement is something that you must do, i.e. you must use gasoline in order for your engine to run.



A recommendation is something that you can do, i.e. you are recommended to use 87 octane fuel. In reality, you can put any octane you want as long as the fuel is the requirement: gasoline.



So if you chose to go by mileage instead of months, no ones going to shoot you.
Definatly do it on miles. They dont wear away when not in use. Seems daft to waste money when they should be ok.
as an owner of a repair shop personally what I do for my own car with air filter and gas filter is replace them once per year weather I hit the mileage or not. the plugs they make to day will last a lot longer than plugs in the past but if they have been in the car for a long time I would change them. I only say this because I have seem plugs that could not be removed from car simply because they were in the car to long. the plug may have rusted,or water filled the hole that the plug sets in and corrosion formed around the plug.
dont go by your manual. any more manufactures tell people that vehicles can go a long time without maintinence. bull ****.

air filters should be replaced 12k or sooner depending on drive enviroment. plugs today can go a long time. large maintinence services offered by dealers. 30k,60k,90k covers all of these but they are pricey.

belts should be replaced if they show signs of cracking.

listen to your dealer technicians recomendations
time only really matters for fluids. They do get worn out just sitting. But an air filter can only get dirty if the engine is running hence sucking in air. Same thing really with plugs. Now wires on the other hand can wear out from sitting because the rubber can deteriorate over time from sitting. Pretty much change fluids by miles or time and things like brakes, plugs, fuel filters, and air filters by mileage