Generally I empty the cup every two weeks, sometimes a less, depending on how much is in the cup, however, like X said, I dont clean the reaction chamber at all.
the cup probably twice a week. the pump/airline/airway path about once a month. I reach in the skimmer and try to clean the top of the reacton chamber every few months. sooner or later I need to remove the coralline algae from up there because the side where the light is has better coralline coverage than my tank.