The amount of time a blower should be operated prior to starting the engine is dependent upon the temperature outside and the amount of time the engine will run.
A general rule of thumb is that if the engine will be operated for 30 minutes or longer, the blower should be operated for 10 minutes prior to starting the engine; if the engine will be operated for less than 30 minutes, the blower should be operated for at least five minutes before starting the engine.