Exercise : 8 weeks to 12 months.

Its a misconception that you exercise your young German Shepherd until he drops.
As the German Shepherd is a fast growing breed the opposite is actually neccassary to help stop irreversible damage to ligaments & joints, also the carrying of to much weight on the overall framework can and will do harm to a young puppy, leading to problems in adult life,

Consider a young Shepherd in comparison to a 3 - 4 year old child, you wouldn't expect them to run a marathon would you?, (Even though the spirit might be willing!!). With growth and Maturity, that time could come.

Prolonged exercise can begin once bones have finished growing at approximately 12 to 18 months of age, but do it gradually . Now having said that your young dog still does need exercise and stimulating, which playing and gentle walks in the garden,park, shops or at the beach can achieve.
Just remember to let him/her rest when he/she becomes tired. (Something you'll have to judge as your young dog will continue performing because they wish to please and keep up with you). Remember puppies need plenty of rest and sleep just like a young toddler.

Moderation & common sense are the guidelines. Allow your puppy to grow steadily & slowly, enjoy their first 12 months getting to know them & teaching them about life in the outside world . Then you can savour a lifetime with a fit and healthy adult.