BMW recommends replacing the engine air filter every 30k miles. I believe the cabin filters is recommended 15k miles. I am sure there is a time element to the cabin filters since they have a charcoal component that absorbs odors, and over time they are going to become saturated with contaminents regardless of miles/use (you can check the maintenance book for specifics). I change my cabin filters annually or 15k miles, which ever comes first. I actually change both at annual or 15k intervals, because the engine air filter is only $10 to $12 online for the recommended brands; Mahle, Hengst or Mann. Mann cabin filters can be found online for about $47 to $50 for the pair, Bosch are about $40. I think the BMW dealer ones are closer to $80 for the set. I have not found a significant difference between them, as long as they have the BMW specified charcoal in the filter.
Just pull them out and inspect.
The cabin air filter, BMW recommends to change once a year if I'm not mistaken.
The air filter should be changed pending on how dusty drives one has. Clean the airbox once the air filter is removed. You can pull out the air filter and eye ball it. Shake the dust/debris out of it and hold it against the light. If you don't see any light at the bends, the filter is clogged and needs replacing.
Also, the color/state of the filter will tell you how bad it is. New it's yellowish. When it starts getting plugged, it start turning into a greyish color and no amount of shaking will get the dust out. Your mileage will start going South at this point.
The cabin air filters, I usually pull out every 2 or so months and clean them (dust shaking). It's a good time to clean the plastic holders and check the drains for clogging.
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