How warm? 2 minutes? Five minutes?
Depends...How cold and did you use a block heater? It also depends on your engine, depends on the personality of your vehicle. I only ever dealt with bigger truck(350, 450, etc, not Semi)
30 degrees...3 minutes.
0 degrees, no block heater(if it even starts) I'd say 7-10 minutes.
0 Degrees, block heater, 5-7 minutes.
You want the engine oil 100+ degrees before you start loading the turbo.
Your mileage will be crap on your diesel before it gets up to temperature. Some of the latest Turbo Diesels probably just do fine with a minute or two warm up. Some of the old ornery trucks I used to deal with probably needed to idle 24x7 :)