Sorry to hear, always hard.
I have an 8 year old lab, he's my best buddy. Goes absolutely everywhere he can with me, and is my best hunting buddy. With all the hours spent training, in the field or just hanging out, we have a pretty good connection. Never have had a dog that I'm this attached to, and he's started to slow down a little bit in the icy water hunts and has become a little bit more cautious in situations that he would have otherwise just gone for it in. But, he still goes nuts when he knows I'm loading the truck up with decoys and the gun.
As for when it's time to put them down... If they are in pain, if their personality changes (like a normally sweet dog becoming a very snappy "don't touch me" type) or when the simple things that they love doing are now a chore, like arthritis so bad they won't go up 3 stairs to retreive their ball. My concern is their happiness, and when they just can't enjoy life, it's time... It's hard for sure and I hate making that call but it's part of being a responsible pet owner.
Hopefully you're concerning adopting a pet that needs a loving home, as there are too many neglected dogs out there that just want to be loved.
My next dog will be another retriever from a good breeder to minimize the risk of any geneticly passed issues so I can have a good hard working dog. If it was just a pet I was after, I would go to the humane society and find a new buddy.