It depends on the severity of the air pollution, and how long and often you wear it. In Europe we would say the filter lasts about 3-4 months if used daily at an average of 6-8h. Doctors and Healthcare people surrounded by COVID-19 patients should change it every 2-3 weeks.
Our research for a very polluted cities like Delhi India has shown that the ­filters should be changed after about only 40-50 hours of usage. The costs for the replacement filters are only CHF 8.90 for a single filter or CHF 18.- for a set of 3 filters.