IMHO 5 mins give or take a few, I've based this out what the game says plus some checking around on max unit speeds. Although PlasmaKrab your timings would fit if nothing is really happening. however as soon as someone moves or fires the 5min rule should apply. All that being said some rounds of HEAT for example should go right though some targets and don't as in real life.
Bob out
__________________ Eternal War(gaming) PanzerBob
"Whenever in future wars the battle is fought, panzer troops will play the decisive role..."
Heinz Guderian, General der Panzertruppe