
The rule that a lot of people recommend is that you walk 5 minutes for every month of the dog's age - so for instance at 4 months old you would walk him for 20 minutes - 4 x 5 = 20. Too much exercise can cause problems with joints so the 5 minute rule helps to try to avoid this and I stick with it until my dogs are a year old.
So often you see people being told that pups will need lots of exercise to wear them out - that's just not true - all it wears out is their joints :-( Mental exercise is just as tiring if not more so. A few sessions a day of training your pup - just a few minutes at a time - will be just as beneficial and has the added bonus of producing a well behaved dog :-)