Poor old WoofDog has been at the vets again today as he has anal adenomas that have now ulcerated. He also has bad arthritis and the start of kidney problems 
He's 13, a Lab/GSD/Newfie cross - DP got him as a teeny tiny puppy, DP was only 19 at the time and street homeless. WoofDog was his absolute saviour and best friend during some really grim times, he's never ever gone without even when DP couldn't even feed himself. DP reckons the fact he had a dog to look after saved his life when he was severely depressed in the past.
And now WoofDog is, lets face it, really rather old and decrepit. We have no wish to keep him going if he has no quality of life - we are not at that point yet (I don't think), though he is pretty much deaf and his vision is going, he does still come pootling along slowly if we go for a walk round the fields (we live on a farm so he has constant access to the outdoors) and he still comes to greet us when we come home if we've been out, and likes sunbathing, and shows great interest in cheese and the offerings our toddler makes from the highchair 
But he is visibly aging and failing and, well, creaky now. He can't hear much, or see much, and can't get up the stairs or into the car unaided anymore. He won't go swimming any more either, something he has always really loved and was still loving last summer. He gets out of breath easily, he spends most of his time sleeping, he is having to wear a collar/cone thing to stop him chewing his arse to bits, and he is now on steroids, antibiotics, and tramadol.
The vet is reluctant to put him through surgery (with the risks of a GA plus the risks of damaging his sphincter muscle) if we can control his symptoms through medication for now, and tbh so are we. DP is very much against what he sees at stringing a dog out when it can no longer do any 'dog stuff' and I tend to agree with that. We want him to have the best quality of life, rather than keep him going for as long as possible by whatever means and at whatever cost.
So... how will we know when its right to end it? Are we sounding really ruthless by not wanting to 'do everything possible'? How do you balance the whole quality of life/treatment options thing? How do I support DP to make the inevitable decisions that are ahead, when WoofDog has been the constant in his life for so long and through so much?