Curious question about mother who has Alzheimer's, I was just wondering how long she can last like this?

My mother is now in a home. She has Alzheimer's. My question is more of a curious one. I cannot have any kind of conversation with her any longer as she just doesn't seem to understand anything I say to her. She gets really confused. I'm not the only one that has noticed this with her. She also seems to be not eating much at all. She rambles on a lot about things that don't make any sense. The other day she was so upset because the kitchen help took her plate away from her in the dining room. She asked me why they took her plate and I told her because she said she was finished eating. She then said she wasn't finished. I didn't respond to that, then just a minute later she asked me what were they going to do with the plate. I said wash it. She then said she didn't understand why they would want to do that. I did not respond to her. This is very common/normal for her talking. She is so confused. I was just wondering when someone with Alzheimers is like this......how long can they last. Someone told me years (she is 93), and someone told me this is a sign she is really failing and they said she wouldn't last much longer. I hate to see her like this.