They should be replaced with filters of the same type and size as the original filter. Media filters will last between 30 to 60 days depending on the filter efficiency. Electrostatic or washable filters should be cleaned at least once every 30 days. You also need to be sure that after cleaning, that the filter has been given adequate time to dry to avoid any problems. Permanent filters should be cleaned and media replaced according to the manufacturer's instructions.
Filters are all different and made for different needs.
A cheap, disposable model may not filter enough dust and pollutants from the air to prevent the system from getting dirty and restricting air flow. The higher efficiency filters such as electronic air cleaner types not only remove a greater amount of dust but also can remove certain odors from within the home which can help those people with allergies and other respiratory conditions, or those who just wish to breathe cleaner air. Some service companies advertise service calls for an unbelievable low price. In today's economy, it is apparent that a company cannot perform quality service charging these types of rates. More than likely, your final repair bill will be much higher.