How long you should let the engine pre-heater be on before starting the car depends on what the temperature is outside.
Engine pre-heaters should not be used at temperatures above +10 °C.
Using an engine pre-heater is a simple way to minimise negative environmental effects. A preheated engine consumes less fuel than a cold one and also produces a smaller amount of exhaust emissions. During short journeys, especially, there is a big difference between driving away with a cold and a hot engine.