While the exact length of time a filter can last depends on a number of variables, a good rule of thumb is to never go longer than 60 days between replacing your disposable air filters or cleaning your washable filters. The state of your air filter greatly impacts the efficiency of your system, so replacing/cleaning your filters is an absolutely necessary part of your HVAC maintenance schedule. Some of the factors that determine the length of time your filter will last are:The model of the air filterThe indoor air qualityWhether the home is occupied daily or notThe number of occupants in the homeThe number of pets in the homeWhether or not anyone suffers from allergiesThe air pollution levels around the home