Some dogs will do fine with a two-block walk, while others can go two miles. A good rule of thumb is that the shorter the dog's legs, the less distance he can go. Smaller dogs, like pugs or toys, won't be able to go as far as large dogs, like golden retrievers. A dog's breed is important as well Chihuahuas tend to be able to go anywhere from 2 -6 blocks. Anything more then that is to much.
Your dog's age and general health are important as well. If your dog doesn't usually get much exercise, you won't want to start her off with a six block walk. Gradually extend the length of your walks to build her endurance. Arthritis, heart disease, and a number of other health concerns could also affect the length of your walks. If your dog has health problems, discuss how far she can walk and how much exercise she should get with your veterinarian.
In the long run, your best bet is to observe your dog while you're walking. When she starts panting excessively and acts tired, then she's walked far enough. This is also a good way to keep an eye on your dog's health. If she used to go three blocks with you and now only walks a few blocks, you should consult your veterinarian. It could be a sign of a health problem.