Discourse
It depends on which site I am updating and when the best time is. on a busy production forum, generally I update/rebuild once a week, but I will also do it sooner if I know a specific update has a fix I want, or if I need to make a change to app.yml (installing a plugin, for example). you probably don’t have to update that often though. on my dev instance I rebuild probably at least every second day.
Docker
I update as soon as it is conveniently possible when there is an update available.