How often do you drain your air compressor?

I have a spring loaded drain screws into bottom of the tank, that opens when pressure is low, coupla lbs.
You can hear it hiss at fire up till it gets a coupla lbs pressure, then it closes.
Now and then it'll surprise you by releasing when least expected.
I try to drain the system low points coupla times a year, my son everytime he paints.