Alzheimer's stages. Is there a rule of thumb for length?

Alzheimers or Vascular Dementia or Lewy Body dementia diagnosis would require brain imaging and blood work and a visit to a Neurologist. Even then, the only positive way to confirm Alzheimer's is after death by examining the brain tissue under a microscope. IF she truly has Alzheimer's, there are specific medications that slow the advance of the disease.
If she has a history of heart disease or stroke or heart failure, vascular dementia is likely. If she has a long history of alcohol use and liver problems, the MD will be looking for ARD with a vitamin B1 depletion that can be treated. So get the tests done and develop a treatment plan to buy time.
How much time varies greatly from one person to the next.

Yes the disease is very individualized. Unfortunately, decline can be seen on a weekly basis as I see with my FIL. Sundowners syndrome is what many people get with Alzheimers. For some reason toward late afternoon or evening they become restless, agitated, and have increased confusion. I do not know what the cause of this is just happens to be yet another issue that comes along with this horrible disease!

Mom has no history of alcohol...but from her first symptoms of forgetting and repeating about a year and half ago, she doesnt remember 5 mins ago, repeating is much much worse, shes no longer able to balance checkbook or follow conversations. Every day, she says 'Im so mixed up today' but isnt able to verbalize what shes mixed up about. Ive noticed she gets angry a lot now. When i visit with her after doing her shopping or taking her to dentist or whatever, she doesnt hardly talk and cant seem to retain anything when i tell her a story about her great grandkids. She has definitely seemed to follow the downward stage progression of alzheimers and has been on aricept for 6 months, altho im not sure that it has helped slow anything. We have an appt with her doctor in a few weeks but i dont want to bring up alzheimers with him in front of her as she insists she just has hardening of the arteries and i dont think she will want to know if its alzheimers.