Does anyone have experience with how long a person can go without food, and only an IV?

My mom is 94, on oxygen, lots of meds for her heart and breathing, and has not been able to walk in 5 years. She is very, very thin. She developed Alzhiemers 6 months ago. 5 days ago she fell out of her chair and broke both knees. The surgeon put pins in her knees to allerviate pain. She is on IV fluids---no food---and will not eat, drink or allow herself to be moved. A person must be with her at all times so she doesn't try to rip out her lines. I think her body is trying to die... She has been on and off hospice 3 times and that is our next step. Thank you.