If your horse was formally diagnosed (thru blood tests) as Cushings and is not on Pergolide or Prascend at his age, he is doing wonderfully
Is he newly diagnosed?
I have a 25 yr old with Equine Metabolic Syndrome, a/k/a "peripheral cushings", even though "they" say the pituitary gland isn't involved
I keep him on Chastetree 365 days with no days off, but I reduce the dosage in the winter. I have been doing this routine since his formal diagnosis in May, 2007.
My thought with your horse is: "If it works don't fix it" until you see something change.
That could be anything from being "off" and not being able to put your finger on what "off" means, to listless, to weight loss or gain, to a change in hair thickness and/or texture, to warmer hooves indicating laminitis.
Smoochies on the snoot to your Grand Elder. Sounds like he is one tough Fella and is receiving great and conscientious care