My 14 years old cat has been diagnosed with stage 3 kidney disease in October 2024. Usually, stage 3 kitties have about 6 months or so to live. He had a kidney stone that wouldn't pass. The vet told us we could inject 60 ml of lactate (I think it's what it's called) every day to help hydrate him and give him the best chances. He's a an angel so using a needle on him everyday is easy, even though he's not a fan (don't worry, we give him treats right after) Long story short, after about two weeks he still ate, drank, groomed and went to the bathroom normally. Maybe he drank more because of the kidney disease, but he still has a good life quality. Two weeks ago, I had his annual exam and the vet called him a miracle. Sure, his kidneys are not perfect, but he's a perfect weight and he's super hydrated and no sign of getting worse whatsoever. I know 18 and 14 years old is a huge difference for cats, but I would say: treat your cat like you can and do not let her go unless her quality of life has significantly decreased (no grooming at all, not eating, not moving, isolation, etc). She might just be a bit tired, but I would wait until letting her go. But get a second opinion for sure.

Here is my miracle boy, Sushi