In sweden they say at puppyobedience class that 5 minutes walkning / month of age.
So if the puppy is 5 months you can walk about 25 minutes just walking in leash/day (It doesn't count the playing and other thing you do with the puppy.)
They say that over here also, but I don't like that rule. I left home this afternoon at 14 hour, and came back at 18 hour. So that was a 4 hour walk with a 5 month old pup. He has run off leash most of the time. We sat down from time to time so I could smoke a cigarette, stopped a few times to let him swim (yes, he does like swimming), let him play with some other dogs who where also walking around there… So he's been active for 4 hours. Not walking next to me, but running around, swimming playing with other dogs. Still he didn't lay down when I sat down. Now he's sleeping next to me, but not as deep asleep as an exhausted dog can do.
A dog, like most other animals, is build to be able to move longer distances at a young age. Look at deer, they run with the group after just a few days. Wolf pups run miles and miles with the pack at a young age. Some birds, wild horses.. run for hundreds or even thousand of miles before they are a year old.
So why wouldn't a dog be able to walk for an hour at 3 months old or so?
I have been looking to find out where that 5-minute rule came from. But can't find anything about it.
But always watch and read your dog so he can tell you when he had enough, and give him enough opportunities to rest if he wants to. Don't force him to do more then he can handle, and try to let him run off-leash, or take on a speed when walking that suits your dog.