There is no way to determine how much longer she will live in misery. Large breeds do not always alert us to just how much pain they are in. She is most certainly suffering. I realize how difficult the decision is, as I have owned many senior dogs with complicated health issues. It is time to let her go. I'm so sorry.
Dont get me wrong at all please. I am a HUGE dog lover but wy are you keeping her alive!???? She is clearly extremely very sick and in pain, why would you want her to be like that, I am sure you love her very much but let her go, she needs rest, needs to be at peace. Dont continue to make her suffer please. If you love her then you would do this for her.