i crate trained my dobie whenever he was a puppy (dobie puppies are the best chick magnets btw). id leave him in his crate for about 2 hours while i was at school but our house keeper would secretly let him out to play around the house .
i never saw it as an issue of time but who and where id let him play. try to take the puppy outside alot and really have him play with children and lots of different people. id recommend taking him to dog parks and/or petsmart so he can be accustomed to strangers. "aggressive" breeds should be spoiled nonstop and introduced to all sorts of experiences before they get older.
i say this because my dobie doesn't trusts kids or bigger dogs. he just wasnt around them very much when he was a pup. also, i was surprised when i was cutting some cedar trees with a chainsaw and he tried to attack it . he hadnt heard a chainsaw before, and it really set him off.
being with a puppy 6+ hours a day is perfectly fine. just make sure he gets to hangout with lots of different people, learns how to (love to) swim early and is around machinery, and he'll grow up into a great dog.
also, german shepherds are velcro dogs. like it or not, she will probably be his "momma" and he'll listen to her moreso than you. that's just how it goes. good luck.