How long does a person last after diagnosis with dementia?

Average 8-10 years: On average, once diagnosed with alzheimer's dementia, the life expectancy is about 8-10 years. However, it can be up to 20 years.

It really depends: If one is diagnosed early on in the illness, it can be a long term chronic disease lasting 20 years. Most people are not diagnosed until they have moderate or even advanced disease so they live less than 10 years after diagnosis.