Walks: How far is too far for a 3 month puppy?

Mr. Knightly is now three months old and we've started doing short walks. Probably the equivalent of walking around the block once several times a day. I know you need to be careful at this age about too much exercise, but he's also acting like he's needs to burn off some energy.

How far is too far for a puppy at this stage? What about as he gets older? We did get the okay from the vet to be out and about in the neighborhood, and I don't let him get in the grass. We stick to the sidewalks.

I want to make sure he gets the exercise he needs, but also don't want to overdo it. He's a Westie and about 7lbs at this point.

Thanks!