My active GSD puppy bites. How much exercise and play to stop biting?

To help keep him occupied when you aren't able to actively play with him, I recommend getting him a Kong toy. My GSD loved his. Also, to save you hands, you need to work with him on learning what is and is not acceptable when mouthing and chewing.

When you play with your puppy, let him mouth on your hands. Continue play until he bites especially hard. When he does, immediately give a high-pitched yelp, as if you’re hurt, and let your hand go limp. This should startle your puppy and cause him to stop mouthing you. (If yelping seems to have no effect, you can say “Too bad!” or “You blew it!” in a stern voice instead.) Praise your puppy for stopping. Resume whatever you were doing before. If your puppy bites you hard again, yelp again. Repeat these steps no more than three times within a 15-minute period. If you find that yelping alone doesn’t work, you can switch to a time-out procedure. When your puppy delivers a hard bite, yelp loudly. Then, when he startles and turns to look at you or looks around, remove your hand. Either ignore him for 10 to 20 seconds or, if he starts mouthing on you again, get up and move away for 10 to 20 seconds. After the short time-out, return to your puppy and encourage him to play with you again. It’s important to teach him that gentle play continues, but painful play stops. Play with your puppy until he bites hard again. When he does, repeat the sequence above. When your puppy isn’t delivering really hard bites anymore, you can tighten up your rules a little. Require your puppy to be even gentler. Yelp and stop play in response to moderately hard bites. Persist with this process of yelping and then ignoring your puppy or giving him a time-out for his hardest bites. As those disappear, do the same for his next-hardest bites, and so on, until your puppy can play with your hands very gently, controlling the force of his mouthing so that you feel little or no pressure at all.

Also, the help keep that busy little mind occupied and teach him appropriate behavior, get him involved in obedience training. Many places have puppy kindergarten classes (if you have a PetsMart nearby, ask about their classes) that will teach you to teach them basic obedience. If he shows an aptitude for it, continue with his training as far as you would like to go. Teach him tricks, teach him tasks, play fetch, whatever you can and need to do to keep his body well-exercised and tired and his mind occupied. A tired dog is a happy dog.