liotte.....this is a tough question. When the quality of life for the animal is no longer good, it is time. If an animal is in pain, it is time.
I disagree totally with what BsO says, in regards to
"I guess I'm selfish like that, and want to keep them around."
That is extremely selfish, and is not in the best interest of the animal. If an animal suffers so that its owner doesn't have to deal with it....it's just wrong.
The hardest thing I've ever had to do is make that decision...but always, ALWAYS I thought of the animal first, and me second. My cat (the longest relationship I've ever been in) is 17 years old....and I'll need to make that decision one day in the not too distant future, and I will not hesitate for a moment, when I know my "Pea" no longer has a good quality of life.
It is ridiculous to set a detective story in New York City. New York City is itself a detective story...
AGATHA CHRISTIE, Life magazine, May 14, 1956