You should change at least once every year, depending on the use. If you have a central humidifier, you should change the water and clean the unit at least once a month. If it's not being used often, then you can wait until it smells musty or there is visible mold before changing it.

Depending on the make and model of your central humidifier, you will likely need to change the hu mate every 2-6 months. To avoid bacteria growth, it is also important to clean the humidifier regularly. Consult your owner's manual for specific instructions on how to care for your humidifier.