When will my German Shepard calm down?

Duke is 2 years old and weighs about 85lbs. He stays outside during the summer because of all the shedding so he has plenty of room to run and play. I play fetch with him for a good hour every night (broken up into increments) but he never tires and cannot sit still for more than a minute without wanting to play again. In the house all he wants to do is play and he's big enough now that he knocks stuff over and scares my 6 yr old daughter. I know he would never hurt her intentionally but she isn't so sure. He never just wants to lay down and chill, he always wants to be playing fetch. He doesn't even want to be loved on or petted very much, he just wants to play. When will can I expect him to calm down?