I am not a veterinarian this is my opinion and experience
 
This is what I have told clients when asked, how will I know when?
 
I tell them to pick 3 things that their animal enjoys.  When the animal
no longer enjoys them then it is time.  For example it may be: eating,
a specific toy, going for walks, playing, taking treats etc.
 
But you also have to be comfortable with the decision
 
Hope this helps
 
Lisa & her 24 paws
[Posted in FML issue 4184]