When is it time?

      She is sick.  We know this.  The vet found a tumor in her mouth/nasal passages which is why our oldest dog is breathing like an old man with a bad cold and sleep apnea.  We have decided (with the vet's agreement) that surgery on a 13 1/2 year old dog is not really a viable option.  We have her on prednisone to try to reduce the size of the tumor and make breathing easier.  Still, she has difficulty if she closes her mouth.  
      Eating is tricky too - she has always had dry food, but she has difficulty chewing it and breathing at the same time.  So, I have been softening her dog food and adding egg, ground beef, and veggies.  She seems to be able to manage this more easily than having to chew.
     So, having said all this, how do we know when enough is enough?  She has some periods of the day that are good and others when her breathing is really rough.  Those latter times are when she will be trying to sleep, then she leaps up, and half stands, half sits, and just stares at us.  She doesn't want to be cuddled and she seems to be tolerating having her back scratched.  She is still aware, and will shift from room to room to be near us.  But how do we decide when to say goodbye to a friend who has been part of the family for over 12 years?
    I am learning that I am not good at this...