In all honesty, this is a question no one, not even vets can give you the answer to. Each case is different and they can go very quickly. A cat with renal failure can live for a few years, months or just weeks after being diagnosed. I've seen cats who appeared in better health than one they were living with die months earlier than one who you had to check was still breathing. He was eating, active and perfectly normal until one night he just went down. Within twelve hours he was gone. In saying that I know a cat who is 17 and is living off 1/4 of a kidney for two years. Main thing is when you notice they are a little off colour to go to the vet, get fluids into them, maybe vitemins ect. Good luck.
