I've idled them longer than four minutes, sometimes when there's been trouble adjusting carbs. And then the hotter it gets, the more likely you'll have to go back and tweak things when it's at normal running temp.
It depends a lot on how hot it is that day. But the thing to be concerned about the most is the cylinder heads. They're the hottest part of the engine.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*