New particulates swirl around in the air all the time. Humans, pets, air-conditioning and heating systems, and open windows are all responsible for introducing contaminants into a room. How long to run your purifier greatly depends on its make, model, and type. Your user manual should give you a guideline, but generally you want to keep it on for between five and eight hours at a time, keeping in mind that you can always turn it back on after it has had a rest.