For electric heaters, it is often suggested to run them for 1–5 hours, though some keep them on overnight, say.
In diesel vehicles, a pigtail-shaped light indicates that the are heating the cylinder heads before starting, usually lasting 30 seconds or less. PREHEAT ENGINE
Generally recommended when temperatures drop below 10°F–20°F (-12°C to -7°C). For electric heaters, it is often suggested to
Are you asking about a or an external heater (like a block heater)? For electric heaters
I can help clarify what "Preheat Engine" means for your specific situation. Should I preheat my Tiffin Bus 450 DP engine? - Facebook