re: Reading dog threads on here for years.... when do you know it's time for the big sleep?

When they outlive their usefulness.
They have to earn their keep, whether that be hunting dog, working dog, or pet.
I see poor old ladies at the Vet with old Fefe, having to make payments of Vet bills out of her Social Security checks. The Vets are so money hungry and dishonest they won't tell her the truth.
I remember while I was in the waiting room, waiting on test results for my beagle, an old lady brought in her dog.
The dog was so old and fat you could not determine its sex from across the room. The dog would repeatedly sit and immediately stand up. He did this repeatedly because it was too painful to sit. He had also lost control of his functions of pooping and peeing. The old lady wanted them to euthanize it. Her, IMO, worthless husband had sent her to do dirty work.
The Vets receptionist tried to convince her to let the Vet prolong the dogs agony.
After the dog was dead, they called the old lady to come back and get the carcass.
I felt compassion for the lady. She had a dead pet and a deadbeat husband.