general rule of thumb, let it idle for 15 to 30 seconds to circulate the oil through the motor then drive sensibly until the motor is up to temp. the reason for not letting it idle longer than about 2 minutes is due to blow by, which is when the oil is seeping around the rings on the piston. the only motors I let idle for long periods are diesels but I would rather have it tached up to about 1200 rpm. gas engines especailly are not good to allow much idle time.