LIGHT-DUTY GASOLINE & DIESEL (car, SUV, pickup): In all but extreme cold, 0 to 30 seconds usually suffices (check your owners manual for any specific recommendations).
• Above freezing temperatures: typically no need to warm-up at standstill; the best way to warm up is to drive the vehicle at a moderate pace.
• Below freezing but above 0 degrees fahrenheit: up to 30 seconds to allow for complete circulation of engine oil. Warm-up time is also determined by the effectiveness of defrosting. Again, driving the car (slowly to moderately) is the best way to warm up in this range. Note that wheel bearings, steering, suspension, transmission and tires also need warming up. The only way to do that is to drive the vehicle.
• Below 0 degrees: in this range, idling the engine for a period of 1 to 3 minutes becomes necessary, allowing thickened engine oil to adequately circulate throughout the engine, and for the likely increased time needed for defrosting/deicing. More than three minutes even in these conditions is considered excessive.
• Remote Vehicle Starters: These devices encourage excessive idling, so in general, avoid using them.
Recommended cold weather warm-up procedure for light-duty gasoline and diesel engines.
Make sure that your vehicle is tuned-up and has a good battery. Except in below 0 conditions, avoid remote vehicle starters which encourage excessive idling. CONSIDER USE OF AN ENGINE BLOCK HEATER which can be timed to turn on one to two hours before starting the engine. They allow less strain on the starter and battery, can lessen stationary warm up time in extreme cold, and allow the vehicle to consume less fuel during warm up driving. Protect yourself adequately from the elements with warm clothing. For non-garaged vehicles, be prepared with the following: window scrapers that work, brushes and spray de-icer. When necessary, clear snow and scrape/de-ice windows before starting. Then start the engine and immediately blast the defroster. In most cases, you'll be ready to go in 30 seconds. SAFETY IS FIRST: if defrosting is not adequate, wait another minute or so.
TIP: For a lower cost and less toxic alternative to commercial spray de-icer, make your own solution in a spray bottle mixng one part water to two parts rubbing alcohol (isopropyl) along with a few drops of dish detergent.
HEAVY-DUTY DIESEL (commercial trucks and buses; school buses): the EPA and several diesel engine manufacturers recommend no more than 3 to 5 minutes. But there are idling reduction alternatives to consider.
• Use electric engine heaters (such as block heaters) to minimize idling time during warm-up, especially in cold weather.
• Install a small generator or auxiliary power unit (APU) specifically designed for a truck that provides heat, air conditioning, and/or electrical power while the vehicle is not in motion. These devices are a better, more efficient alternative to idling as they use substantially less fuel and emit less pollution. Depending on fuel cost and the amount of time spent idling, the payback on these devices can be six months to a year or more. EPA SmartWay Verified Idling Reduction Technologies.
HOW LONG SHOULD A WARMED UP, PARKED VEHICLE IDLE?
LIGHT-DUTY GASOLINE & DIESEL: According to EPA SmartWay Vehicles and EPA Consumer Information, the overall recommendation is 30 seconds. Actually, any more than 10 seconds of idling uses more fuel than is required to restart the engine. However, the break-even time to offset any potential incremental maintenance costs to the starter or battery is 30 seconds. So, as a guideline, if you're stopped out of traffic for more than 30 seconds turn off the engine.
HEAVY-DUTY DIESEL: EPA Clean School Bus USA and several diesel engine manufacturers, and EPA New England and the American Trucking Association recommend shutting off unless there's a need for turbo cool-down after high-speed, high-load operation. Engine manufacturers also recommend to shut down and restart at least several times a day rather than continuous idling to save fuel and engine wear. Generally, fuel consumption during engine start-up or restarting of medium- and heavy-duty engines is equivalent to about 30 seconds of idling.
TIP: Keep blankets or afgans in your vehicle to use when parked in cold weather to help avoid excessive idling.
 Possible exception: if vehicle occupant(s) is very elderly or an infant in below freezing temperatures.
 Light-duty diesel engines similar requirements; check owner's manual