I had this same question while riding my bike with my dog, how fast was too fast? The dogs will let you know. My girl kept up a great pace for awhile, faster than I thought with how the wind felt in my hair, but she was still in the Shepherd trotting gait and not an actual run, so it couldn't have been that fast. I slowed it down when she started to look tired, and ending up walking the bike 2 blocks from home. She loved it! We've done it a couple more times since then, and she gets all set and ready next to the bike and tries SO hard to keep a perfect heel until we get our speed set and a good rhythm going.
My boy Shepherd doesn't go as fast, he has a bad hip on the right side and doesn't have the body build to go as fast as she can in that Shepherd gait. So with him I do half the speed and 1/3 the distance I do with her. The puppy isn't far enough into her training to do it yet, so I was thinking about getting a treadmill for myself and the dogs on rainy days. The boy dog has supplements he has to take for his joints, and he needs light exercise on top of it to keep him fit and pain free. Poor guy, he's only 2 years old and we have to treat him like an old man.
But for a young, healthy, fit dog, tire them out so long as it's safe. I'd imagine you want to watch the speed on a treadmill much the same as a human work out... start out slow, increase speed, maintain that speed for a bit, then slow it down for a cool off at a walk. A dog can trot for a good while, out right running in short bursts. I didn't want to tire my dogs out with the bike so we never went faster than that working type trot. But on a treadmill safe at home, I could get them running for a short time.