At what point is euthanasia the right choice?My cat is sixteen and…

At what point is euthanasia the right choice?

My cat is sixteen and declining. She still eats and uses the litter box, but she's lost weight, sleeps constantly, and seems confused sometimes. The vet says her kidney values are getting worse. I don't want her to suffer, but I also don't want to give up on her too soon. How do you know when it's actually time?