Depends on the locale, the type of dog, the walking habits, physical health, and acclimatization--and WIND. And the controlling limiting factor is usually ME and not the dog, who would be otherwise content to lie in deep snow in 20-degree weather. But, we do our four-mile walks in nearly all cold extremes, with doggy booties only coming into play at 10 degrees or lower temperature, when the PAM sprayed on the feet no longer does the trick.
But, yeah, wind. That makes a tremendous difference, not only in the perceived temperature, general "comfort," but ability to regulate body temperature in subfreezing conditions.
But this is when the acclimatized, physically active dog is continually moving and under supervision. Sending a dog out to the backyard in extreme cold, snow, ice where they are not, can be a totally different issue.
Heat is a different story. Like Rorey upstream, we go for long walks pre-dawn in the hot months and then nothing major until way after sundown. Like becoming vampires. 
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