Making the decision to put a pet down is difficult and consulting with a veterinarian can help an owner make a decision, but generally a dog should be put down when it is in too much pain to live a quality life. There is no right or wrong answer for when a dog should be put down and the owner will have to make the decision based on the way that the pet is behaving and feeling.
For patients with terminal illnesses, should owners focus on creating the happiest, healthiest and most pain-free solution with the dog's veterinarian. However, there may come a time when medication is no longer working for the dog, such as if it sleeps all day long, each day and is too weak to play games, or if the dog no longer wants to do any of the things that it loves or if the dog often whimpers in pain. When a dog's quality of life drops to this level fora significant amount of time then it may be time put the dog down.
The best thing to do for all dog owners is to consult with their veterinarian and to closely watch their dog from week to week. The most compassionate choice is always the best choice.