How often should I clean or replace the wick/filter of a cool-mist humidifier?

SOURCE: mold in vicks cool mist vaporizer

if you can take it out side and plug it in, put some bleach in it and let it run for a few minutes. then wash it out and run the rest of it out.

SOURCE: Cool Mist Humidifier Filter Drying Out

Do you have tried to use without filter?

SOURCE: cleaning

For Model V745A Vicks Warm Mist Humidifier:

Weekly Maintenance (Removing Scale)

1. Turn the switch to off and unplug the corn from the wall.
2. Remove Water Tank and empty any remaining water from the water tank.
3. Remove Medicine Cup from the Cooling chamber from the Base. Carefully remove the Cooling Chamber and Removable Water Tray from the Base.
4. Soak the Removable Water Tray in a solution of white vinegar and water for 20 minutes. Rinse thoroughly and then wipe dry with a soft clean cloth.
5. Fill the heating chamber with undiluted white vinegar. Let stand for 20 minutes. Dampen soft, clean cloth or sponge with undiluted white vinegar and wipe out Heating chamber to remove scale. DO NOT use soap, detergents or abrasive cleaners.
6. Rinse the Heating Chamber using clean warm water, being careful not to submerge the base in water.
7. The Water Tray, medicine Cup, and Cooling Chamber are top shelf dishwasher safe.

SOURCE: humidifier not working

Ensure there is water in the tank.

• This may be caused by mineral deposits in the water channels. Try the following: Clean the water tank, base and vapor nozzel with a damp cloth to remove any build up in the water channel.
  
• When was the last time the cartridge was changed. (If it’s been over 3 months, this could be the reason why the unit is not working properly.)
  
• Unscrew the demineralization cartridge and run the unit, if it starts to mist then you need a new cartridge. (insert cartridge link here)
  
• The screw cap for the water tank, onto which one first screws in the demineralization cartridge, has a small rubber gasket. make sure that gasket is not too far down against the outer screw cap. If it is the spring loaded button would not work properly to let the water descend into the base tank .
  The solution: push up on that small rubber gasket so that it is as far up and away from the screw cap as it can be without falling off, and voila! you get mist!!
  
• If you have changed the filter recently then take out the cartridge and run the unit without the cartridge for about 15 to 20 minutes.

SOURCE: I have a Cool Mist V3500 humidifier. The fan

TRY POURING IN SOME WHITE DISTILLED VINEGAR AND RUNNING IT. THE VINEGAR LOOSEN UP ANY SCALE AND JUNK. AFTER RUNNING UNIT FOR AWHILE, DRAIN OUT ANY REMAINING LIQUID AND ADD DISTILLED WATER OR FILTERED WATER. RUN UNIT AGAIN. THEN DRAIN OUT ANY LIQUID. NOW THE UNIT SHOULD WORK AS NORMAL. IN THE FUTURE, TRY USING DISTILLED WATER OR FILTERED WATER.