I have read that from diagnosis to death it’s generally 4-10 years with dementia. How does one know what stage the patient is in...early, middle, late?
https://www.dementiacarecentral.com/aboutdementia/facts/stages/
Also have a look at Alzheimers.org which has a lot of good information and coping tips about dementia/Alzheimers in general.
Best of luck!