I wish it were as simple as you'll know when the time is right...but it isn't. It is a personal decision and sometimes our hearts get in the way of the right choice. An old friend of mine had an 18 year old scottie who meant the world to her. That dog had walked her through some of her toughest times and shared some of her best times. When it was time to let her go she couldn't see it. She did everything she could to keep her girl going for a little bit longer. One day I gently but firmly said that her time had come. I knew that she might hate me for saying it but after the fact she thanked me. Sometimes we love them so much that we think that we can still help them when infact it is time to let them go. She was so afraid that she was giving up on her scottie too soon, she felt guilty thinking about letting her go because what if there was something else that could be done.
I felt the same things when I had to let my Hutch go. He was my boy and I lost a part of my heart the day I let him go. Reasonable logic goes out the window when it comes to something that you love so much. Sometimes you'll know it is time, sometimes you need a great support group to help you see through your love to see the truth.
Just enjoy every moment you do have!