It depends on filter to filter. Most pre-filters need to be changed every six months if they are non-washable. The washable filters can last you for a little longer but over the course of time they too, would need to be changed. Most carbon filters come with one pre-filter pre-installed. Some even come with two so that you can save on the cost of buying a new filter. However, pre-filters do need to be changed or else the effectiveness of the carbon filter will be hampered.