Keith writes:

Q1. Do I have to drain the air from the tank after each use?

No.

Q2. If the air doesn't have to be drained, should I bleed the water
from the drain cock after each use? every week? every month?

Draining water is a chore, and doesn't alter the saturated conditions
slowly corroding the inside of the tank, so monthly would be overdoing it.
Draining doesn't actually dry the insides. It should last as long as you
need it no matter how often you drain it.

http://www.truetex.com/aircompressors.htm