Did you know that a dirty filter cleans the air better than a clean one?
The dirt that gets trapped in the filter actually help to attract other particles as they pass by due to electric charge and the filter's pores closing up (so to speak).
The problem is reduced air flow, once you get past a certain point there is not enough air passing through to cool the compressor resulting in loss of cooling and eventual compressor failure.
This is why it is almost impossible to create a AC filter that cleans like a room filter as air purifiers do not have to perform much cooling (just a fan motor in this case).
So it's a give and take thing. A dirty filter cleans better than a new one but (after a certain point) at a cost.
Both equipment and exposure (from changing the filter) need to be considered when creating a filter change schedule.
No way to recommend a specified time frame due to differences in dust accumulation at any given location. It's something you'll have to decide for your particular unit and location. I let mine go until it's pretty dirty (sometimes 60 days or more) and have not had any problems so far (10+ years).
With your readings you need to seriously consider air purifiers.
Click below for more
https://www.netc.com/bb/viewtopic.php?f=10&t=1157#p6719And remember that you can "run these units into the ground" before a filter change and simply throw the whole unit out as they do not require the cooling that the AC unit does.
Good luck..