How often do you drain your air compressor?

My compressor is mounted upstairs above my work area, not easily accessible. The drain is plumbed into a copper line, which leads to a 110v NO valve, like this:
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The compressor is on it's own dedicated switched circuit, which also powers the valve. This way when I turn the compressor power on, the valve closes; and when I turn the power off, it opens and lets the tank drain (the valve is connected to a hose that goes outside). I think the valve cost me under $20 and has worked well for many years.