I'm so sorry, watching a pet decline isn't fair and is the worst part of having them. As cliche as it sounds, you will know. One day she will look at you with a look that she's ready, and you will know that it's time. I'm not going to say it will be easy, because it won't. In fact it will be heart wrenching, but I would rather lose my best friend a little too soon then make her suffer because I'm not ready to lose her.