How often do you drain your air compressor?

No one's mentioned an air dryer. My work threw out a perfectly good 115-volt air dryer. Now I just need a shop again :( I know it just treats the air after the tank. I like the drain on the timer idea though.
Ingersoll Rand D25IN, Non-Cycling Refrigerated Air Dryer, 15 CFM, 1-Phase 115V