In general, a normal household with an HVAV system should change its air filters at least once every 90 days. Having pets ages your system at a faster rate. Check your HVAC filter every couple of weeks to see what state it is in. If it's dark and dingy, replace it with a fresh filter. In addition, your air ducts should be cleaned every three to five years.
Additional air filters around your home can also help limit the bacteria and dander floating through the air. Tabletop filters work well in smaller rooms while standing air filters are effective in larger spaces. Filthy air filters do more harm than good. Make sure to buy additional filters for each filtration product you use, so you can change them as frequently as needed.