Question about Xiaomi Smart Air Purifier 4 Compact

Unfortunately, the air filter can only be used once. But it lasts quite a long time.

I do blow mine out with an air compressor every now and then. It does not reduce the digital readout of the remaining life of the filter (in the app), but it makes the airflow improve slightly.
The normal life of the filter is probably well over 2 years. I ran mine for a few (3 or 4) hours a day for the first 9 months, then I changed to leave it on permanently for the past 6 months. It still shows 73% remaining.

Single use only. You need to buy a new filter every 9-12 months.