I think at this age, he'll let you know when he's had enough..
I personally, don't do alot of heavy exercising at this age, I take them for walks, hang out in the backyard, do some tugging, playing, but nothing to strenuous..
You can usually tell when enough is enough..They'll just flop down and won't move
Be careful of the heat,,it is HOT HOT here in CT, and tho my shepherd would hang outside /go for a hike all day long if I asked her, I limit her outside time, it's just to darn hot..
You can't keep them in a bubble, but I try to avoid any jumping, hard surface long walking ,
Congrats on your new puppy soooooo CUTE!