HVAC (heating, ventilation and air conditioning) filters should be replaced once a month during high-use seasons like summer and winnter, and once every three months during low-use seasons like spring and fall. You can easily tell if they're ready to be replaced when they look dirty. Dirty air filters can cost you $200 - $300 more a year in energy costs since the unit has to work harder than it would with a clean filter.
Photo Credit: brighton-co-plumbing.com
Source: Greg O'Keefe, co-founder and COO of MyEnergy