I had to put down my Choc lab a couple years ago. He was 13 years old and I had him since he was 6 weeks. I don't think I will ever have another dog as good as he was. He had been having problems and started getting sores all over. I had him to the vet a few times and nothing really seemed to work. The day before Easter he just looked at me like he had had enough. It killed me, but I did it for him. I wanted so bad to back out right before the vet put him down, but as bad as it hurt I knew it was only fair to him to help him out.
I really miss him, but I have no regrets about doing the right thing for him. I just knew it was time. You will know when, and as much as it hurts think about your dog, and how much suffering he will endure if you don't do what needs to be done.