What it a person's life expectancy if they have late stage Alzheimer's?

I am a daughter and caregiver of my mother who is 88 years old, final stage of alzheimer's, meeting 5 out of 6 of the criteria on the list by carol. Mother is in good physical condition - no major medical issues and intact skin and maintains a good weight. She is on TPN and has speech and occupational therapy twice a week. She is starting to take one meal a day by mouth and doing very well. More than 90% of the time she doesn't know who I am. But there are those few times when she does know me and that is why I stay with her at home. Is there some kind of time frame after a person gets to late stage, final stage alzheimer's of their life expectancy? I have asked this question before and never ever get a definitive answer. But there has got to be some average???? My sisters and I want to keep Mother at home as long as we can but also have to think about the cost involved. Of course it is more than an issue of money. All of our lives are changed by this disease. Some sort of plan would be nice but it seems that Alzheimer's is also usurping any carefully laid plans by its victims and their families. Hope this question doesn't sound like whining and hope you have a substansive answer. Thanking you in advance for your reply.