Jessie, who is 13 3/4, has kidney disease. She's on even more meds than Harold, plus fluids every second day.
After she had a bad patch in July, I talked to my vet about how I would make the decision. I'm watching for three bad days in a row (she rallied after 2 1/2 in July), total loss of appetite (I've jumped through all the food hoops), or vomiting (they can develop ulcers in their stomachs similar to the ones Jessie had in her mouth before we increased the frequency of fluids). So far, she's always had the light in her eyes; if she loses it, that will be enough as well. At the moment, she's doing well thanks to the anabolic steroids we have her on for her appetite (they've also improved the strength in her back end). She pants a lot, her eyes water, her nose and mouth drip, her urine is almost colourless, her breath is awful (teeth are OK), and she's a bit wobbly. But she's always happy to go for a walk (shorter and slower than they were), loves her trips to the vet, is currently eating fairly well, loves any and all attention, and is quite a happy camper.
I hope Harold has some good time left, and that he gives you a clear sign.