When is Water getting OLD/Stale

Since city water has a lot of chlorine in it, after my yearly sanitization/dewinterization pass, I will drain the tanks after a trip and putting the rig into storage, but otherwise, I try to keep them full, because you never know if you may need the water. Since the sanitization process kills bugs, I don't hesitate to drink or shower from the tank water.

I could be wrong, but I'm not dead yet (hopefully.)