DIAGNOSIS The Blue Pure 211+ doesn't have a smart filter indicator light or timer. It's up to you to monitor it based on time, air quality, and performance.
FIX Go by the calendar and your nose. Blueair recommends changing the main filter every 6 months for typical use. Mark the date you installed it. If you have pets, smoke, or live in a high-pollution area, change it every 3-4 months. The other sure sign is a drop in performance. If the fan seems weaker on the same setting, or if odors aren't being captured as well, the filter is likely full. Finally, do the 'light test': take the filter out in a bright room. If you can't easily see light through the white pleats, it's done.
IF THAT DIDN'T WORK If you're changing filters on schedule but still feel the air quality is poor, make sure you're buying genuine Blueair replacement filters. Some third-party filters don't have the same construction or filtration media and can be much less effective.
HEADS UP Don't forget the washable pre-filter! Clean that fabric shell every 1-2 months. A dirty pre-filter will make the main filter clog up way faster because it's not catching the big hairs and dust first.