She told me what was HER limit.
The first puppy I adopted as an adult on my own - I took on the responsibility of her well being with the knowledge I would be with her until her last day - and I was. At age 2 they told me she had a slipping knee cap -- it bothered her to go up/down a lot of stairs -- $15K operation to repair. OR live on one level and it wouldn't hurt her quality of life. I moved to a one story home, always assisted her up in to bed and made a ramp she could slide down go get out on her own. She did GREAT.
At 10yo she became listless -- I took her to our regular vet and found out her life was at risk -- she was in ketoacidosis from adult onset diabetes -- we went straight to the emergency animal hospital. She was admitted for 5 days of constant IV etc. I didn't even hesitate -- no matter what it cost -- because she was still "herself" and many people live with diabetes. $6,000.
Following her release, I had to learn to give her insulin shots twice a day. All the insulin and syringes -- I did so religiously for three years. The diabetes blinded her -- and she was still "herself" -- she adapted and I adapted (wore jingle bell bracelet at home so she could follow me etc. -- and she knew our neighborhood and house already -- so she could run etc. It was still a really good life. She was still herself.
But as she slowed on her walks... and then could only make it around the "little" circle trail, not the big one on her walks... I knew the time was coming -- and I knew she would tell me when. She did.
She was always the pack leader (I had three by then) -- and she stopped asserting her position. She stopped sleeping up on the bed -- which she'd done for 13 years. I realized, she couldn't hear the bells anymore... and she was... in a dark, quiet unhappy world. Her decision that she was done was sudden, but very clear if you were looking for the signs. She was my first true baby.
I knew -- and I cried all the way to the vet. They had lots of measures they could take -- options to try -- tests to do -- but she had always hated unfamiliarity of the vet. It wasn't the price that mattered then... or the fact that I didn't want her to leave yet... you're never ready. It was the thought of my now anxious baby who couldn't see or hear much anymore -- and would have been poked and prodded by people she didn't know, couldn't see and i couldn't calm her with my voice. It would have been torture.
I sat down, with her bottom in my lap, her paws on my shoulder and her head laying against mine. I held and pet her in all her favorite ways for a long time. Then they quietly administered the final shot. I felt her heart stop, the breath leave her body... I was her mommy to the very end -- a safe, quiet, peaceful, comforted end.
I've never been so proud of myself for doing what was right but it broke my heart and soul into pieces.
The priority MUST be quality, not quantity, of life. If they are still themselves... even with some changes and adaptation -- I will do whatever helps them keep their comfortable happy life. But I won't ever cross the line to keeping them going beyond because I can't let go.
I hope my story helps you discern the right path in some small way.