This is an old wives tale that just won't die. The radiator is called a radiator because its job is to radiate heat, not cool water. The hotter the radiator is, the better it works, and the way to keep it hot is to pump as much hot water through it as possible. There is no time involved, just the rate of heat exchange.Get a good thermostsat like a Robert Shaw (I've had good luck with them). With nothing there, and a high flow water pump, the coolant can be flowing too fast and not giving the radiator enough time to remove the heat from the coolant.