Originally from what I remember, cabin filter change recommendations were time based, but now are distance. That may be reasonable, but can be problematic in some situations like say you drive lots of dirt roads, or often get some really polluted air whether from pollen, smoke, diesel fumes, or dust.
They're kind of a pain to take out to just look at their condition. Laborwise, just as cheap to put new ones in, but maybe not if you're doing it yourself.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above