Hi richard, I'm sorry I have no idea about the length of time it will take but if you feel her time has come, then just let her go. I don't think it will be very long. DNR seems sensible. My mum passed away in April, no dementia but diabetes. She started to pass out now and then, we had a chat, her, us children, the nurse in charge and her GP and she said she was all for DNR. She was 93. One morning she was gone.
My husband passed away in June and he stopped eating and drinking the day before. Cancer, no diabetes, no dementia as he was only 55.