Anything over 5 or 10 minutes, I shut it off and here's why.
Don't worry there won't be a test later.
Most diesels especially the older ones, didn't stay hot enough at idle for complete combustion partly due to overcooling. Without complete combustion they would start puffing smoke and could even start misfiring. This incomplete burn would cause glazing on cylinder or liner walls, which takes hours off of an engine.
The newer motors are better in this regard and maybe there is no problems with the RTV maintaining temp but I still don't let mine idle for to long especially when it's cold out.
Years ago we would encourage our drivers and operators to bump the idle up if they didn't want to shut their engines off.
In fact the newer (computer controlled) OTR trucks have idle shut down features on them that if enabled will shut the engine off if the throttle sees no movement in a preset time. This is for several reasons: incomplete burn, emissions and economy. The driver can override this feature by bumping the idle up, usually with the cruise control buttons that double as a type of hand throttle.
While many diesels might not be harmed by prolonged idling, they sure weren't designed for it, they were built to work and they don't work at idle.
Starters are cheaper than engine work.
Hope this helps.