how much do i exercise my german shepherd puppy?

If your puppy is only 8 weeks, obviously he can't go out off your property yet. He has to have his complete course of vaccination shots before he can do that. Normally best practice is not to do much 'controlled' exercise (roadwork) for the first 6 months, although obviously all puppies need to be taken our for short walks, and free runs in a safe area, to get used to their collars and leads and to start to build up their muscles to support the immature bones and joints. Between 6 and 12 months, you should gradually increase the length and amount of time you exercise for, until by 12 months, hopefully if all has gone well with the development of your puppy, he can probably take as much as you are prepared to give. Common sense really

But the person to ask for the correct advice for your puppy, is his breeder!! Only they know what works for their breed, and specifically their bloodline. Provided the person you bought your puppy from was reputable, listen to them.

Play with him inside until he wants to sleep. It's not rocket science.