How do you know when to give up - on a sick kitty

We’ll he is eating, but not drinking enough, that’s why we have to do the saline.
As far as his quality of life. It’s hard to say. The vet says cats are good at hiding pain, so it’s hard to tell.
I’m sure if we stopped the saline, he would get dehydrated and probably stop eating and would probably pass on his own. But doing that would be just as hard to watch as putting him down. Guess there’s just no easy answer unless he starts showing signs that he’s in obvious pain.