I bought a timer made for out door use(good for X-mas lights etc), it saves on electricity as well as wear and tear on the heater. I would think that how long you leave the heater plugged in somewhat depends on how cold it is. From what I have seen in Alberta during some bitterly cold weather, people leave their cars plugged in at work all day and that includes ALL the cars not just the diesels. I always though it kind of looked like the old west when people would tie their horses up except now its cars(more pick up trucks than cars though )I've got one of these in my '85 300DT as well. I've never had to use it yet. I'm glad that I read the posts about it. Now I have an idea how long to keep it plugged in if I ever need to use it. I always thought you kept these things plugged in over night.