I battled this dilemma recently with my beloved dog, and there are no easy answers. He did not tell me "it was time" - however, he was losing weight and strength; then one leg grew more and more uncooperative, until it was finally my fear that he would fall and REALLY hurt himself, that made me make that awful decision. I'm sure if you asked HIM - he would have said something about the other 3 legs sort of worked and he was OK with that..... sort of like the French Knight in Monty Python's "Holy Grail" who keeps losing body parts and still hurls insults at his foes....
I guess there just comes a point when each person says "I love you too much to see you live like that"