I let my truck run for 30 seconds when it is cold or two minutes when it is very cold. In the summer, I let it run for 15 seconds. I was reading an article two years ago that claimed that anything more didn't accomplish much, apart from warming up the interior and defrosting the windshield. (Obviously, if things are frosted over, I wait longer for things to clear.)
When I first purchased my truck, I let it warm up for 10 - 15 minutes or until the engine temperature was 100C. The dealership recommended I do this until I put on the first 2,500 km. It may be why my fuel economy is higher than most others, I do not know. Everytime the fuel economy issue comes up, I think back to how I broke in the engine and if one thing comes to mind ahead of anything else, it is the warmup time.
Just my two cents.