Depends I guess
When I light it off I am not on the ground long after that. The easy thing to do is sit a small (good quality to avoid fires) heater in the aircraft and leave it until ready to fly. We do that at night in the winter to keep drugs warm and it also prevents the fogged up windows. When warm enough for the drugs but cool out at night I walk out to a helicopter with condensation all over the windscreen. Kind of a pain.