When is it time (nine lives are up)?

From your description, she is no longer "living"--just existing. Yes, it's time. Make sure all the family gets to say a final goodbye, and hopefully at least one of you will want to remain with her until the very end (but not if it would just be too difficult). It's tough, but I always feel the final kindness I can give is to be holding them and talking to them as they drift off to Rainbow Bridge....