I honestly can say that as long as they were having good days and not in pain I would wait for a bad day.

With Angus he was getting worse, but by no means was he ready to go before he did. I just got up one day and his leg and foot was swolen. The vet did everything she could but his poor body just couldn't fight the infection so we made the decision to let him go. It was a bad day, but that boy was so happy to see us when we got there that morning. We just petted him and talked to him and he slowly faded away.