I just really like clean glass, so everyday or more frequently is best. And it's quick, because there isn't time for anything to really build up except a bit of condensate. When I smoked a bong I could go weeks without a water change. Gross Dabs seem to produce more condensate on the glass. That amber/yellow stuff just looks gross even though I know it's harmless. Of course with every cleaning comes an increased chance of breaking/dropping the glass.