OP, hugs and my sympathy, I can completely relate…
Warning - this is long, but since it is the exact same situation, my hope is in posting it vs. pm, that it may help someone down the road as well.
I lost one to chronic kidney failure two and a half years ago. He was 21 and I had owned him for 20 of those years. He had very vague symptoms the prior couple of years before the diagnosis. Not sick, not lame, just not himself. My initial thought was Lyme, but he tested (Cornell test) negative. Since it sounds like your horse is going down the same path mine did, here’s somewhat of a timeline at the end:
Mid June - vet recommended running the standard CBC bloodwork to check for anything out of whack since the horse had been mildly NQR for so long. Kidney values weren’t good. Not horrible at that point, but not great.
Mid July - recheck showed significant worsening. He also had a tremendous amount of plaque build up on his teeth - the vet explained its relation to the kidney failure, but I don’t remember exactly what. By that point his deteriorating condition was noticeable. I asked the vet how long he thought the horse might have. His answer was anywhere from 3 to 6 months up to a year, maybe two.
First three weeks of August - by this point it was obvious the horse was not going to have a whole lot longer. He had dropped a lot of weight and he had no appetite. I had him on daily dex and a full tube of ulcer guard. At first it helped a little, but after a short time it didn’t seem to do anything. I did give him this until the end. He was grouchy. He didn’t want groomed, the only thing I could use on him was a soft brush.
Third week of August - I made the call to the vet and scheduled that final appointment for the next week.
After two years of vague symptoms without a lot of change, he went downhill very fast at the end. You may experience the same with your horse. The vet may have given you an amount of time, but don’t expect that to be the case.
I wish I’d have done it sooner. But, even after five decades of owning horses, this was the first time I’d had one with kidney failure. While I knew he would not get better, I was desperately searching for any information I could find.
Hoopoe’s first two lines are right. While I couldn’t stay away (I own my farm), the last few weeks I woke up every morning with a horrible sense of dread. So, please allow your feelings to help guide you. And, the number I saw was 75% kidney function LOSS by the time symptoms are bad enough that a diagnosis is sought.
As all the other replies have said, sooner is much better - especially with this condition as the chance of a sudden rapid decline is very possible.
I’m so sorry you’re going through this.