Like so many questions this is one that the answer starts with "it depends". The moisture in the tank comes from the humidity in the air. If you are in a dry climate you rarely have to drain it. If you are in a humid climate, you'll have to drain it more frequently. My advice, start at a week and if you are getting nothing out go to a month, and on and on until you determine for your location what is appropriate. Unless of course you get your jollies draining the tank and in that case, go for it every day.