Question about hospice and the length somebody can live with no fluids...?

My grandmother is on hospice, and hasn't had any fluids since Thursday. Her feet are starting to mottle, and they can't get a BP reading on her and can only feel a weak pulse in her carotid artery. Since yesterday she hasn't opened her eyes at all. She hasn't experienced apnea that much. How long could somebody live without fluids and how do you know when the end is approaching? Are there any signs? I want to be with my grandma, and I have been with her everyday all day. I just get an uneasy feeling every time I leave the hospital...and a lot of sleepless nights afraid of that dreaded phone call. Thanks!