How long should I warm up the engine before starting casually riding it in the city for keeping the engine as healthy as possible? Not at all, 30sec, 5 min or longer?
And how long NOT to warm up please? In case it is not advised to keep the engine idle for a period of time.
This question is specifically for the R1200GS LC if that makes a difference.
A minute or so is usually fine
Unless your riding in sub-freezing temperatures of course, then you'll want to let it warm up a bit, but if not. I wouldn't worry much about this.
The LC version of your BMW boxer motor has considerably tighter tolerances than the air cooled version as it does not have the temp/heat variance as much as an air cooled engine has.
This means, it's more modern and less prone to piston rattle and what not and can be run much sooner than an air cooled engine.
When warming up an engine I'm typically touching the cylinder and ensuring that it's getting hot. After a minute or so of running on LC engines, I feel comfortable.
Occasionally, I'll just start them up, do a few short rev's and take off.