I don't have a specific idle or shutdown rule that I follow, I just do what seems right at the time. Idling for 30 minutes at 1600
RPM's, I don't do. Running a diesel hard and shutting down immediately, I don't do.
If I have been running hard, I will allow the diesel to idle a couple of minutes before shutting down, especially if it has a turbo. If I haven't been running hard, I may let it idle for a minute. These usually apply to ending my work.
I see no reason to idle for 5 minutes with intermentant stops, normally. But take my old backhoe for example. I may shut it down every time I get off for even a minute or two because of it's lack of good brakes and hydraulic leaks. Given the same situation for the Kubota, I would let it idle for a minute or two.
A diesel engine needs to warm up before operating under heavy load, and needs to cool down afterwards before shutting down. I would say anything over a couple of minutes, you should shut it down.
Diesel locomotives are to be shut down if idle time will exceed 15 minutes. The time limit may have been less, except they figure it will take approx. 5 minutes to crank it. On the other hand, they are not to be shut down within 30 minutes of operating at wide open throttle.
UPS wants their trucks shutdown at stops, but it may also be for safety.