Diabla is a 10 or more. I haven't seen her tired of fetching yet, and when I had a cold and had no energy to play tug or to train I only played fetch with her and it could be more than an hour every time.
If I don't give her something to fetch she "fetch herself" trowing stones back with her front paws and running to catch them, sometimes even in the air. She HAVE to be always doing something, don't expect of her to be tied calmly to a tree, she will find something to play with or she will make it.
Even in our Monday trips, after 3 or 4 hours walking and climbing she is constantly grabbing stones, just to have something in the mout,h so I have to have always a ball or kong with me to discourage playing with rocks.