My Grandmother Has Been Put In The Care Of Hospice

My grandmother has been put in the care of hospice because doctors have said there is no more they can do to treat her gangrene. She was going to have a leg amputated earlier this week but my family was told her oxygen levels are too low, and if the surgery was performed she most likely wouldn't wake up again. We've been told that they can keep her medicated and make her as comfortable as possible until her time comes. If gangrene is fatal and a patient has been moved to hospice about how long can we expect before she passes on? In addition to the gangrene she also sufferes from some heart failure (clogging around her arteries) and there is also fluid on her lungs.