In a worst-case scenario, if a home or building owner were asked if he or she changes their filters, the answer would be "What filters?" In a best-case scenario, the answer would be "Like clockwork, every 30 days after I pay the utility bill."
In reality, the most common answer falls in between the two scenarios. According to industry recommendations, filter-changing intervals can be based on the pressure drop across the filter. Changes can also be determined by a calendar schedule ranging from every one to six months, or by visual inspection. Frequency of changes also depends on the indoor/outdoor environment and the health of building occupants.