There are a lot of variables and not an answer fits all.
Type of breed?
Region of country?
What is the dog used to?
Indoors or Outdoor dog?
If you can keep them dry and out of the wind, they can withstand a lot colder temperatures than we realize, I think. I have a black lab and am much more concerned about heat than cold.
Here's kind of my guidelines I've followed with mine...
In kennel under truck topper, I have a cheap indoor/outdoor thermometer so I always have the readings in the cab with me. You'll be amazed how quickly it can heat up under there.
80 degrees - start watching temp because it may not take long to get into 90s.
90 degrees - he's not back there.
Cold - I've never had to worry about it. Kennel with an insulated cover over it and their body heat keeps it warm. When on hunting trips and he sleeps in it overnight and it's down well below freezing, there's condensation in the topper in the mornings and steam coming out of his kennel when I unzip it in the morning. One time it got down to -20 air temp, -45 wind chill overnight, and I also wrapped a thick heavy shipping/moving type blanket over the kennel and he was very warm the next morning when I let him out.
In outdoor kennel/dog run at home.
Heat - no temp is really too high as long as you can keep them in an area that has proper airflow, shaded, water access.
Cold - I follow the windchill, 0 degrees in the morning I usually don't think much of it if it will warm up soon as the sun comes up. Prolonged periods of negative windchills, full day, etc., then I might keep him in the garage or do garage in the morning, then outdoors in the afternoon. I put tarps around most of the run to block the wind, insulated dog house with straw. Keep them dry and out of the wind, and there's not much to worry about.