What is the life expectancy once Alzheimer's or dementia is diagnosed?

Now I read that people with ALZ tend to live longer than people with Dementia. Even though ALZ is under the Dementia umbrella, it effects the brain differently than other Dementias. I read that Dementia runs 5 to 7 years. In hindsight, my Mom showed signs before 2011 when she had a bad head injury. The injury was Aug and she passed Sept 2017.

My Aunt lived for 12 yrs after diagnosis with ALZ.

It depends. ALZ progresses more rapidly in those who get it at a younger age but those who are older may have their life limited by various comorbidities, and different types of dementia have different timelines... and don't forget that diagnosis may take place years after symptoms first begin, so that has to be part of the equation.
Here is a website that I think gives a good overview:

https://www.unforgettable.org/blog/what-is-the-life-expectancy-for-someone-with-dementia/

It really depends on what kind of dementia someone is dx with. Vascular dementia (VaD) a person can live 5 to 10 years. Alzheimer's person can live for 20 yrs on average. This numbers are from Alz.org and Mayo clinic. This does not mean someone with VaD will die in 10 years that person can die sooner or live longer just depends on overall health.

I think that's a hard question to answer since many people die of old age in general.