I get confused w/ him cause 1 minute he looks miserable and the next he's chasing pennies I throw him!
I'm sort of going through this with one of my 20-year-olds right now - obviously he really has had a long, mostly happy life! (My sister has said she wants to be reincarnated as one of my cats because of the care they get.) But he's had chronic kidney problems and chronic pancreatitis for 7 years now, and last year developed severe dental disease because I didn't want to anesthetize him for cleanings (and didn't think he'd last long enough for it to become a problem - BIG mistake on my part).
If I'd decided to put him to sleep when he was having one of his acute pancreatitis attacks (Aug 2008 or Dec 2010) it would have felt right because he was just curled up in a ball with that faraway look of pain in his eyes. (I'd be much more aggressive with painkillers now after a couple years on the assisted feeding & pancreatitis Yahoo groups.) But I syringe fed him to keep him nourished & alive and the attacks passed, he resumed eating, and he's had more good years.
But now the dental disease - which I've kept at bay for a year with antibiotics - is no longer responding, and most of his body is fine, but he stopped eating again and when I syringe feed him, if the food goes to the wrong part of his mouth, it obviously hurts. I switched antibiotics and it seemed to help, then regressed. I'm picking up another today, probably my last try. If this doesn't work, I have much the same case as an oral SCC cat - a cat who, most of the time, is happy, but not when he's fed. He runs from me now when he sees me coming with the oral syringe. When do you say, this is enough?
Unfortunately, this is all part of loving little furry animals . . . We can only give them the best lives we can while they're with us, and use our best judgment (and that of those we trust) at the end.