Ive made the decision twice. First was when I was young ~12 years old, one of our family dogs, Bailey, had cancer at 8 years old. Was more my dog than anyone else in the family, so parents let me make the call. One day she wouldnt get up out of her kennel, and when i was petting her she put both paws in my hand, which had never happened before. Took that as a sign that she was saying to me its time.
Second time was my dog Jake, who I raised from a pup when I was 20. Had to put him down at 12 years old after multiple health issues over the years. He had been having some seizures and a bad cough towards the end. I could have put him down multiple times and did not because he still fought to live as best he could. He would mainly lie around sleeping most of the day, was happy just to be pet and lay on the porch sniffing the air. Great hunting dog, best nose Ive ever seen outside of hound dogs. Something happened one day internally and it was obvious it was time. He was going to die slowly in my arms in pain, or quicker at the vet. I picked him up in his bed since he couldnt walk (100+ lbs) and drove him to the vet to be put down. Stayed with him to the end. Either way its the hardest choice to ever have to make if you treat your dog like family.
Im just thankful I was home that day to say goodbye and take him myself. When people say you will know when its time, its true. Just a huge variable though on so many levels. Make a choice and dont regret it. Our friends have short lives and dont get to be around with us forever. Feel for ya.