How often to change Aprilaire AC Filter
9-12 months is a recommendation. The only way to check a filter accurately is with a dual port manometer

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Nobody is going to buy one of these just to check the filter. Visual and auditory cues are usually enough. If the pleats on the filter are starting to deform it's probably due for a change.
Also keep in mind that ceiling returns tend to pull in less dirt, thus increasing the life expectancy of your attic mounted filter.
All things considered I wouldn't be surprised if you get 3 - 4 years out of the filter. It's dependent on usage and you'll need to monitor it. But don't hesitate to leave a clean and functional filter in because it's old.