I would let it run for about 5 seconds (max) and then take off. I think the whole warm up your car bit is something that was promoted by "Big Oil" to, well, sell more oil/gasoline. A lot like the 3,000 miles per oil change, very unnecessary and wasteful. As a result, the supermajors get more money. It may not seem like a lot of oil/gasoline, but when you multiply it by the number of cars on the road it really adds up quick.