My insight is that you'll know it's time months after it's time. Obviously totally a individual decision. At some point you'll know. IMO in retrospect you realize you've delayed the inevitable past the time when it would be better for the dog not to have delayed.
Let's fact it, it's a horrible decision. I think Zande has given some great advice in this area based on a lot of experience. But again, it's always going to be up to you. Playing God turns out not to be much fun.
Not a lot more I can offer other than my heartfelt empathy.