How long will a person with dementia live if they won't eat or drink enough fluid?

My mother is in the late stages of dementia. She has been barely eating anything for the last few months, but over the last couple of weeks she barely eats at all. Saturday she ate 3 grapes, 3 small pieces of watermelon and 1/8 of a ham sandwich. Yesterday she ate a piece of toast. She has got to a point where she is so weak she can't get out of bed and has become incontinent. There is no more conversation, she just sits in the hospital bed and stares at the ceiling. It is so difficult to watch. My Dad, her husband of 61 years, sits at her bedside all day trying to get her to eat or talk, but she responses less and less. I feel like she has given up. What can we do to help her? Should she go to a hospice or a dementia care cottage? Last week she could dress herself and take care of her personal needs, but this week she can't. What could cause the change?