When the MRI was done, was a volumetric study done? I think that's the most important and most informative thing. It tells what the total brain volume is; how much atrophy there is, and where the atrophy is occurring (what parts of the brain). Then, if another MRI is done later on, with another volume study, you will see what the progression is. The second most important thing is a good Neuropsych Evaluation.

Timelines are a VERY gray area; no two patients are alike, so no two people will have the same progression. That said, you might want to look at the information provided on this website (duration of each stage):
https://www.dementiacarecentral.com/aboutdement...

And you might want to use this calculator:
https://eprognosis.ucsf.edu/calculators/index.php

Life expectancy is hard to predict since each individual is very differerent even though behaviors can be similar. My husband has had mixed dementia for 10 years.

The PET can show WHICH parts of the brain are affected . ThT plus the neuro/psych battery of tests can pinpoint which of 20+ mental/thinking functions are problematic. A lumbar puncture can also be done, telling what beta amyloid plaque and tau tangles are doing, in the Cerabral Spinal Fluid, CSF, for short!

The MRI should confirm for your neurologist where the atrophy is on his brain. My husband also had a PET scan and a spinal tap in the early diagnosis stage, along with testing from a neurologist psychologist who made the initial assessment of Posterior Cortical Atrophy as his variant of Alzheimers.

Life expectancy is not something any of them can predict...but I was told out of the gate to get my finances in order. That was 5 years ago and I am still not completely in order!

Jay got diagnosed through a geriatric psychiatrist. It's extensive testing, and he was pretty far along when we got started with it and he couldn't do much of it, but the diagnosis was rock solid. No experience with a geriatric neurologist though.