According to http://www.outsideonline.com/1856881/20-best-dog-breeds-runners, a Rhodesian Ridgeback is the breed of dog that's the best for endurance running because it was originally bred to hunt lions.
There isn't really an answer to that. Someone might be able to make a rough guess, but you'll have to provide the parameters of this scenario.
For instance, is this a healthy adult dog? Where is the running taking place? On a flat African savannah or through snowy mountains? Is the dog supposed to be in top physical condition, active house pet condition, or sedate life style.
Even if you name all the conditions, it still varies from animal to animal based on their heart. Not their physical heart, though that too, but their will to keep on going.
It's probably a long time, though. They say wolves can spend days running down prey and sled dogs pull heavy sleds through the snow all day long for several days during events like the Iditarod.
My Rhodesian is 2 years and 4 months old , by 18-20 months is when he reached his peaks , or at least peaked for what we have pushed him to . He will run 8 miles while we bike from our home to the downtown of our city, then we rest for an hour or so rehydrate , eat , and bike back 8 miles. The last two miles or mile he starts to slow down, but keep in mind as stated in the comment above his enviorment plays a huge role (his health/condition like humans does as well). He runs on side walks, back streets & there is an uphill in route and a up hill street on the way back too . We’ got our bikes in June and rode well into October . The cooler days he definitely had an easier & faster cool down. He keeps up with a bike pace which is faster than running . If it’s really hot it’s mean to make them do long hauls . I always pack plenty of water . Post hot runs he will get home cool down drink water jump in our lake and lay down to recoup on easier cooler long hauls he does the same cool down but could honestly probably go do another 4 mile run with me after a little rest . My boy is 110 pounds maybe bigger now healthy very lean . My girl is 8.5 months 80lbs she’s a chunkers but doesn’t do very long bikes or runs yet 2-4 miles tops Amazing breed ! Hope this helped .