I called half a dozen mechanics across half a dozen states and got recommendations that ranged from 0 seconds to 10 minutes.
Driving Tips for Winter in Indiana
Winter car prep: A mechanic's advice on getting your vehicle ready
Reasons Your Car Is Hard To Start After Sitting, car starting
Winter Is Back, but Don't Idle Your Car
Warming up your car in cold weather can cause engine damage
Should you warm up your car before driving in cold weather?
How Long Should You Let Your Car Warm Up in Winter? The Answer May
How true is this? The rest of the article states starting it and
Should You Warm Up Your Car Before Driving?
Warming up your engine by idling does not help your car – Chicago
Let there be heat: How long should you plug in your car, car plug