To warm or not warm is the question? The temp has a lot to do with the answer. The colder it is the longer you should let your vehicle idle. Below 20% I would suggest pre starting the engine and let it idle for a minute for every 10% below 20% IE at 0% 3 minutes at -20% 5 minutes. There is no need to idle your car till it heats up, comfortable yes, but not necessary. If you park in a garage and more so if it is attached these times may be shortened but this is a good rule of thumb. Above 20% by the time you hook up your seatbelt, check your mirrors, set the radio and make sure your cell phone is on mute is plenty of warm up time.Idling does not do any damage or shorten the life of an engine. Today’s fuel system’s have total control over the amount of fuel your engine is using and will not let it run to rich. If your check engine light is on then it is a different story. Some believe synthetic oil would allow shorter times but not only does your engine have to warm up the transmission and power steering and all the other parts in general have to warm up. Techs in New York during the recent blizzard did some research on an idling engine. A 3.8 GM uses .5 of a gallon of gas when idling from cold start for an hour. A 4.7 Chrysler, like in my truck, uses .6 of a gallon for an hour. The only question I have is I believe these were performed in a warm shop. If they were performed outside I believe they would have been slightly higher. One last tip if you use auto start set your HVAC system on one lower then high fan and defrost when parking at night. There are also relays available that will kick on heated seats and rear defrost. Sounds like overkill to me but those heated seats sound comfy.