I've had more than my fair share of gliders lost this year due to what I think is periodontal disease.
I said I think because the vets will not give me an actual diagnosis or treatment that actually works. The closest to a diagnosis they have come from testing is the symptoms are bacterial infections. But they can not or will not tell me a cause of the bacterial infections.
We have been to 3 different veterinary hospitals/offices seen 3 or 4 different vets with varying degrees of experience with gliders.
The other day I lost the 9th glider to same thing, all different ages and backgrounds. One of the 9, Lemmy lived for almost 2 yrs showing symptoms another, Amy lived a year with symptoms. Others showed no symptoms until about 2 or 3 days before passing some as little as the night before. So I know what your going through with long term medical care.
Right up to almost the end for Lemmy he had a very good quality of life despite the symptoms showing. He ate well, played normally, etc. At the end, his quality of life started to decrease. Much like you, That is when we started discussing putting him down.
I happened to be out of town at the time so for the first time my boyfriend had to be the one to determine if he had or didn't have a good quality of life anymore. From what he described we decided he had been through enough and it was coming whether we wanted it to or not and we didn't want him to suffer. Up until this point I personally don't feel he suffered. At any rate we decided it was time so I called and made an appointment but the soonest we could take him was 2 days.
Darrin watched him diligently and described how he was acting and I started preparing Darrin that Lemmy wasn't going to make it to the vets. I've been through this soooo many times I can pinpoint within 15 min of losing them. Darrin did his best and kept him hydrated and warm but after a very long 2 years the meds and infection had taken it's toll on Lemmys body. He did not make it to the vet, but he was not alone. Darrin held him and loved on him the same as I would have if I had been home.
This infection can hit reallyy hard and really fast, most of the time there isn't enough time to take them to the vet to prevent any suffering or decline of quality of life.
A lot of times at least with this undiagnosed issue they are looking and behaving normally right up until the night before they pass. So their quality of life hadn't decreased over any length of time. The night before they look and act normal, full of energy and life then the next day lethargic and barely hanging on.
It really hits me sideways sometimes because it can be so unexpected yet others carry on for so long when they have clear visible signs they aren't well.
Quality of life is my answer to your question. When the quality of life isn't what it was, and if it continues to decrease rather than improve, with little to no chance of improving......then I believe it's time, and it's the most unselfish thing you can do for the animal if the animal doesn't make the decision for you first. I also believe the animal knows how difficult that decision is to make and knows your struggling with it so they make it for you.
My one blessing through all this is I've been fortunate enough to be here and hold and comfort every single glider as they have crossed that we've lost except Lemmy and he had Darrin and wasn't alone either.
Every single loss wrecks and destroys me. Every single loss breaks my heart a little more. But I'm ever so grateful to be present for all but one. I can tell as they say goodbye I can tell when they are fighting and when they give up I can tell when they are no longer feeling any suffering even as their body vibrates from a seizure, I can see in their eyes their body is still shutting down but the spirit is already gone and they no longer feel it.
Grief is the price we pay for love. We invest so much of ourselves into these gliders that it can be a very heavy price to pay indeed.
I'm so sorry you have come to the place where you need to ask when is enough. It's not an easy decision to make but as long as you put his best interest before your own it will be the right decision for you both no matter what you decide.