I've always based it on whether they're suffering in some way, seem to be in pain, or are no longer able to enjoy aspects of life.

If your kitty isn't suffering and still enjoys eating, being petted or groomed, watching birds, etc., then she may still have some quality of life.

It might be nice if you could get an assessment from a vet, who might even tell you that she's in "comfort care" territory, but it can be so tough to find a conservative vet for that sort of thing.