This is a hard problem. I have been there several times.
My advice is to let your cat live for as long as it does have a quality life, and I think it is a good thing to be prepared to take the decision of letting your cat go when the time comes.
But for as long as your cat does eat and, drink and function normally, it is best to let her live until she starts to have problems.
In the late stages of kidney failure, toxins buildsbuild up in your catcat's organism. This dodoes affect the brain and she might be confused and have other problems.
When this happens, it is best for her to be put down and it is best for you to let her go.
You do not normally need an appointment at the vet for this. Your vet will understand how hard this might be for you.