A mentally stimulated pup who also receives the proper physical exercise he needs will be a happy pup. But how much is that exactly?
Young puppies have different exercise needs than adult dogs. Activities like riding a bike should be avoided until 12-18 months of age (larger breeds finish growth later than smaller breeds).
Personally, I started walking my Rottweiler pup at the age of 8 weeks. There is a rule of thumb that says 5 minutes exercise twice daily per month of age. Some people even split the time between walks and training.
Keep in mind that the level of exercise you will give your puppy when he is young will define the energy level later on to a certain degree.
Tip
Vets recommend that very young puppies go on short 15 minute walks with several play sessions a day. Exploring and playing is far more important for a 3-month-old puppy than walking on a leash.