I have a 16 yo dog who I check to make sure she's still breathing every time she sleeps for a long time, and it's been almost three years since I had to let a very special 15 yo dog go. I knew it was time after our vet had done everything she could do (including chiro, acupuncture, and several holistic supplements), and things that used to help didn't help anymore...and my sweet dog was struggling to eat, drink, get up to let me know she had to pee, and needed help standing up to go potty or she would end up falling down and peeing on herself. (BTW I came back to add she was not at this point for long - she was doing fine with the treatments for a while and then took a turn for the worse over a couple days. IMO it would not be kind to let an animal go like this for very long.)
She was a smart dog with dignity and this was no way for her to live...hungry and thirsty, waiting for me to help her take care of these things, and needing assistance to accomplish the most basic bodily functions. She was tolerating life, not enjoying it. It was time.
We made one last day that was set aside just for her...held her, petted her, talked to her, fed her treats, took lots of pictures, laid her in a sunbeam and rubbed her belly - everything she loved that she could still enjoy. There were lots of doggy smiles and tail wags. It was a good day. Our vet came to our house and she went to sleep peacefully in my lap. It was hard to let go but really a relief for everyone, especially her. We all felt good about doing this for her.
I know it is very hard, but you will know when you have done everything there is to try, and it is time to let you furbaby peacefully go on his way.