Interesting thread, for me its when the quality of life would be such that basically you would keeping then alive for yourself - the hardest part of ownership is making that decision, we have lost 3 ponies, 1 Natural causes, 1 from an allergic reaction to a drug and not chance to reverse and 1 where it meant we knew that to treat would have an effect on his welfare and possibly cause more suffering without a positive prognosis of a recovery, we could have treated and the outcome be the same with the stress for him that would have gone with it.
We have Tess now at 29 who has long term health issues with her COPD combined with those of old age, this year she suffered the loss of her companion, was depressed and combined with her condition, age and excessive heat lost a lot of weight, we looked at her and tbh putting her out onto 3 acres of grass and giving her Baileys NO: 15 was our last hope to see if she would turn a corner, as she was we expected having to make a decision before winter. Well we are pleased to say that its worked , she still has her COPD that will not change but on her 6 monthly check last week the Vet was pleased with her considering , her COPD has not worsened and she is putting on weight, she is happy grazing around the field with no pressure on her and was quite happy to make him walk after her before allowing him to examine her in her time. She winnies when you go the field again and walks briskly to the fence and is interested in what is going on and almost licks the surface off her feed bowl, she talks to her friends over the fence when she feels like it and goes off on her own when she wants her own space, she has really turned a corner over the past 2 weeks, we know a time will come when we will have to let her go but she has decided for us its not now.
She's Family, we don't move them on when they are no longer of use or fulfilling the purpose, we are lucky that we can afford to keep them without expecting anything other than the love and friendship they offer us in return for an awful lot of money/time and worry but worth every ounce/pound , theres many a time over the past years I could have given up on life had it not be for a hairy friend to cuddle and cry with.
for most owners to make the decision to Let them go, PTS, Kill them however you want to say it is the hardest decision to make, not taken lightly and hurts like hell, not to be judged IMO.