This varies with type of dementia and the age of the person when symptoms are noticed; however, on average, the life expectancy is believed to vary between 8 to 10 years from when the first symptoms are observable (The Alzheimer Society of Ireland, 2019).

References:

The Alzheimer Society of Ireland. (2019). What is Dementia and Alzheimer’s? The Alzheimer Society of Ireland. Available from: https://alzheimer.ie/about-dementia/what-is-dementia-and-alzheimers/