It all depends on how dirty your air is. I typically get about 2-3 months. Ir you're considering it, I would recommend the following fan.

https://www.homedepot.com/p/Lasko-20-in-Power-Plus-Box-Fan-B...

I have 3 running in my home and they haven't been turned off for over 1.5 years, still going strong. The filters I buy at Costco when on sale (3 for $29).

This one has been on the fan 24x7 for the last 2 months and is still good for a couple of more.

https://imgur.com/JfSjbbg

As another commenter noted, it depends on your air, but a decent HEPA filter (H13/H14 etc.) will last a very long time if you remove the coarse dust first with a prefilter.

The prefilter is typically a lot cheaper and is replaced more often. Even some non-woven filter fabric (like that used in vacuum cleaner filters) will dramatically increase the HEPA filter life, is super cheap, and can be washed for reuse.

My purifiers use an F8 prefilter and H13 HEPA filter, and the HEPA filter will last about 2 years.

Filter class info here:

https://www.emw.de/en/filter-campus/filter-classes.html

We run box-fan filters during wildfire season here in the PNW and change them out after the season ends. Last year they were quite gray after all of the fires. We run Honeywell HEPA cleaners year round and bring the box fans out to supplement as needed.

Normally they should last a year, but a solid week of 200ug/m3 is enough to clog one. Speaking from experience.