How much exercise a dog will need will depend largely on it's age, size, health, and breed
Your dogs breed will influence the level of exercise it needs. For example high energy breeds such as terriers and Huskies will require more exercise than say a Bassett hound or St. Benard.
This is important when choosing a puppy as to considering how exercising your dog will fit into your daily lifestyle.
Age is an important consideration in the exercise regime of your dog.
Puppies generally have a lot of expendable energy which they use in short bursts so shorter more frequent walks or play sessions may be more beneficial leading to longer sessions as your dog develops.
As with people as your dog reaches the more senior stages of it's life it is important to adjust the exercise regime to shorter sessions at a slower pace.
In all cases an active dog is a more healthy dog in both weight and mental stimulation.
It is worth noting that all dogs are different and each will require different levels of exercise taking into consideration it's background and character.
Your dogs health is important when determining an appropriate exercise routine.
Balanced diet and regular exercise will help your dogs energy levels and active lifestyle though if your dog dog suffers from a medical condition such as hip, heart or respiratory issues it may be advisable to restrict exercise routines to a level that will not cause discomfort.
In every case if in doubt consult a vet for the best advise.