When I let my dog off leash to chase tennis balls and other dogs I just watch his tongue to see how hot he's getting. He'll start out with the thing in his mouth and when it gets to hitting himself in the back of his head with it I'll get him to take a break. It's a great indicator as to how hot and exhausted a dog can get.
I would think a dog's level of stamina would be different from dog to dog with some breeds doing better than others. I've always thought a dog doesn't know when to stop for his own good if he's running for something he really wants. On a hot day with little water they can push themselves too far.
It should be up to the dog how far or how hard you run and it's up to the handler to not let him go to far.
