The type of whole house humidifier that you have will determine its exact routine maintenance schedule. In general, however, whole house humidifiers require much less maintenance than standalone humidifiers. Standalone humidifiers have a much greater risk of bacteria and microorganisms breeding as a result of standing water. To avoid this, you must regularly empty and clean single room humidifiers.
In many whole house humidifiers, you are only required to replace the humidifier pad once a year. It is common to have this done whenever you have your annual furnace maintenance performed by a service professional.
If you live in an area that has hard water, it is often recommended that you clean and dissolve mineral buildup every couple months. For the recommended care of your particular unit, consult your manual.