People write to me all the time for advice about when to euthanize. They confuse my being a dog lunatic with my being a dog expert. I'm very reluctant to offer any kind of counsel. First of all, I don't know their dog and haven't seen the situation firsthand. I know only what they're telling me, and that could easily be colored by the emotional state that they're obviously in. All I can do is give them the benefit of my experience, and tell them how we've evolved on the subject.
The most important clue, in my eyes, is whether the dog is eating. Now, I don't mean to say that if it's not eating that means its life should be ended; I mean it in reverse. If a dog is in bad pain or feeling miserable, it will not eat. So I don't think we have ever made a decision to euthanize a dog that was eating well.
The other rule we go by is less well defined. It involves dignity, and our absolute refusal to let a dog lose it. If a dog can't get up on its own, if it is urinating on itself, those are the kinds of things that involve a loss of dignity in our eyes, and we just don't think that's fair to the dog.
But it's almost always a tough call, and the bottom line is the quality of life. The best advice I can give is to try your best to think only of the dog and its quality of life. In most cases, if a loving dog owner is struggling with the decision, then it's probably time to let the dog go, because those are the kinds of owners that look for reasons to delay and deny. It's human nature.
Sometimes you get it right and sometimes you get it wrong. All you can do is your best.