There are many factors that can determine when an air filter should be changed.Ā How often the HVAC unit runs, air quality, humidity and other factors can vary greatly from application to application. Ā For most 1ā, 2ā and 4ā deep panel or pleated style filters, it is usually recommended to change the filter every 2-3 months.Ā In many commercial and industrial applications static pressure can be monitored and the filters can be changed when they reach their recommended final resistance in inches of water gauge.