Sorry for taking so long to reply, not been online for days.
My cat was fine until about 1 month before PTS, she had been lethargic and off food so took to vet for check up, vet discovered very large lump on kidneys (probably tumour, but not appropriate to investigate at her age).
She went downhill rapidly from then, lost weight, slept beside food dish and hardly went anywhere. She was still eating (little) until the last day or so but very little poo, drinking lots and copious amounts of dilute wee (sign that kidneys are really not working).
For the final week or two I kept thinking it would be that day (that she would need PTS, didn't want her to suffer) but then decided to leave it until further deterioration. IMO, as long as she was able to walk (a little) to eat/drink/use her tray and still seemed interested in her surroundings then to leave her be. On her last day, as said, found her lying in own urine with no attempt to get up, knew it was time 
Best wishes & hope this helps somewhat