I clean out my humidifier at least once per month or twice per month depending on how often I need to use it.
I mostly use my humidifier in the winter months when the furnace and heating are on and the air in my home and bedroom get dry.
Dry air can lead to coughing and even bloody noses because the dry out dries out the nose and blood vessels in the nose which can then lead to nose bleeds.
The humidifier also helps when I have a cold or flu and have congestion.
During the winter I keep my humidifier on most times while I'm home to keep the air most and after a few weeks I usually clean the humidifier out or at least once per month when I get to it.
The owners manual that came with my humidifier said to clean the humidifier every 3 days but that's too often for me.
But you can clean out your humidifier as often as you like but as long as you cleaning it every few weeks to every month depending on how often you use it then you'll be fine.