How long does the end of life process with dementia last?

My mom is 85 years old has severe dementia and blindness. She is DNR and was taking off all meds other than Comfort Care. She developed a severe kidney infection and was put on end-of-life hospice about a month ago for 3 weeks now she does not eat barely drinks anything and I was told she would probably pass within a few days 3 weeks ago. Today she rallied a bit and actually ate a little bit and was more aware. For the last two weeks all she has done is sleep and babble incoherently if you do wake her. She has lost a lot of weight and is surviving solely on perhaps a pudding cup and a glass of juice daily. How can someone survive on so little for this long?