I have a friend who's advice of when to let a pet go is something like this, "I would rather they go one day too early, than to suffer one day too long." I am on this side. I don't want any animal to suffer and because of human selfishness, many owners drag out the process until the animal's quality of life has simply disappeared.
I found this article and it has lots of good points in it. It is written by a veterinary oncologist which simply means animal cancer doctor.
Check it out.