A Labrador puppy should not be over exercised, in fact when you first acquire your pup the exercise he gets from playing with you and his visits to the garden for toileting will be adequate.
Puppies need much less exercise than fully grown dogs. If you over exercise a growing puppy you can overtire it and damage its developing joints, causing early arthritis. A good rule of thumb is a ratio of five minutes per month of age(up to twice a day)until the puppy is fully grown, i.e. 15 minutes when three months old , 20 minutes when four months old etc. Once your puppy is fully grown at eighteen months old, he will be able to endure as much exercise as you are able to give him.