this is a hard problem i have been there several times.

my advice is to let your cat live for as long as it do have a quality life
and i think it is a good thing to be prepared to take the desision of letting your cat go when the time comes.

but for as long as your cat do eat and drink and funktion normally it is best to
let her live until she is starting to have problems.

in the late stages of kidney failiure toxins builds up in your cat this do 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 apointment at the vet for this,your vet will understand how hard this might be for you.