How accurate is the filter durability that the normal filter <-> air purifier tracks? If it tracks reasonably well (which, without more info, I have my doubts for the same reason printer ink cartridge capacity tracking is bad), wouldn't using these stickers kind of defeat the purpose of using the filter in the air purifier, since these kinds of filters do have finite durability and after that it isn't purifying the air as well as it could be.
I set up several air purifiers at the same time in December 2021 but used them in different areas, i.e., with different filter loads. The air purifiers ran 24/7 and mostly in automatic mode, except for some hours after known possible coronavirus exposures.
Official end of filter life was reached after about five months and for all filters on the same day. The air purifiers are still running tough, that's a plus!
Xiaomi recommends to replace filters every 6 to 12 months. The recommendation, however, is based on the use in polluted Asian cities, I guess. We have rather clean air here. I therefore assume that it is safe to use the filter for an additional few months.
At home I use 3 air filters in various rooms. These are standard/mid-range Japanese Sharp filters. Japanese because we live in Japan, not because Japanese HEPA filters are special. The filters themselves can be bought for about 30$ online. When the filters get near end of life I've had success by switching to a higher fan speed. This is not magic, at some point even on the highest setting airflow starts to match the old medium speed. The high speed mode consumes about 4x the electricity of medium speed.
The net result is there exists an intersection point where continuing to use a filter costs more money than replacing. For us in Japan with expensive electricity this point exists sometime after the airflow has diminished but the filter is viable on high speed. In cheap electricity countries the filter might become unusable before electricity becomes a significant cost.
May I ask why you're using them at all? Every time I look at Japanese cities they seem to have PM2.5 levels in a second-digit microgram range. A bad day seems to be something like 15 µg.
Candles, cleaning, frying. All very good ways to increase air pollution in your home that nobody speaks about.
Unique to Japan is the pollen. After WW2 Japan was reforested in mass with a single variety of tree. Thus making the exact pollen release an annual torture test for anyone without air filters. My theory is everyone eventually gets pollen alergies, it's just a matter of time. My wife is affected during pollen season, and I've starting to notice myself become affected as well.
We do not stir fry, that's not a thing in Japanese cuisine. And our cooking is all IH so no gas particle. Yet even we will notice if the air purifiers are not running. We'll first notice our noises are a bit stuffy, and omly then check to find a kid has turned off the air purifier.
An upshot to low pollution is filters last ages.
Xiaomi filter processing Canadian air in a semi high traffic area after 2 years looks better than my Asian country filter after 6 months.
When on auto, it is mostly spinning at idle, hardly sucking air through the filter, and as such it's hardly contaminating the air filter (in fact the airflow is so low I stopped using auto mode at all, there is just no point).
It also doesn't take into account how contaiminated the ambient air is (even though it has an air quality sensor on the one I have).
I run mine at about 40% so I use the filters about 2-3 lifetimes :) Even that is a pessimistic approach IMO because the air here is very clean. I mainly have it because of hayfever (pollen allergy).
You are of course right that automatic mode does not do much. At the same time, higher fan speeds are noticeable (and annoying).
I know the sensor works because once there was a small bin fire outside. There was no visible smoke indoors, I only noticed a slight smell. But the purifier immediately ramped up and showed over 200ppm.. So it does work.
I'm surprised it's so low as I live at the 2nd floor on a fairly busy street with many diesel buses passing.
OTOH a lot of the Xiaomi purifiers have built-in air quality monitor so really you can just ignore the useless filter health value and figure out if the filter is working for yourself, based on how it performs.