If you had a pet that had a chronic health condition that required more vet visits than the average, at what point would you put the pet to sleep?

What factors would you use in deciding whether or not to put a pet to sleep (euthanize)? At what point would you say “enough is enough” and decide it’s time to end the pet’s life?

I have a cat with a chronic condition, but I am deliberately keeping this question vague, as I don’t want it to be about my cat and her issues specifically, but just want to know in general what other Jellies would use in their decision making regarding this issue.