This is a bit hard to answer exactly because different dogs, even among the same breed, will have different exercise requirements based on activity level, health and other variables.
However for young puppies, several good play times and romps in the yard will be enough. The young pups should get enough exercise on their own to suffice without having to add in extra work.
Through adolescence the pup will need more exercise. However it's very important to stress that until the growth plates are closed in a puppy - anywhere from 10 months to 2 years depending on the size of the dog - strenuous forced exercise should be avoided. For a normal sized Lab, growth plates should be closed around 16 - 18 months.
Growth plates are the soft plates between joints that don't harden until the puppy is done growing. While we may think the pup is finished growing, the bones may still be developing, so the plates are not yet closed.
Once the growth plates are closed, more vigorous exercise can occur, although it's important to always watch for signs of injury, lameness or over exertion.
For an adolescent pup, exercise can increase and can include short jogs, games of fetch, swimming, etc. Again, exactly how long the exercise should last will be dependent on several variables. For instance, swimming is actually quite a bit harder than a jog, so a swim session should be shorter. However be sure the exercise keeps stress off of the joints. For instance in dog agility, most trainers won't allow adolescent pups to start jumping, weaving or doing other joint stressful activity until the growth plates are closed. However, tunnels, low contact work and other flatwork is encouraged during this stage.
Some of the potential dangers of over exercise are stress related injuries, tendon and joint injuries, emotional stress on the dog, heat stroke, early arthritis, etc. Carefully watching your dog's body language will tell you if you are over exercising or not. Any change in exercise tolerance should be discussed with your vet.
