How long do dementia patients live once they become delusional?

Doctors several decades ago used to say that after any diagnosis of Alzheimer's Dementia a patient had 5 to at most 8 years to live. As that has proven absolutely mistaken, they seldom will "guess" in this day and age.

How are you defining delusional? Because, that can start at any point with dementia. My husband sees vapor trails, or thinks there are extra cats in the living room. I dunno, to me it's just part of the disease. My mother had nice chats with an invisible guy-she had end stage cancer.
If anything, I am watching for physical decline in my husband-less mobility, sleeping more, weight loss, apathy, and so on. Cognitive decline is a given with dementia-and the delusions (so far) do not bother me at all. For me, it's a peek into the way they see the world and it's ok.
There really aren't any terminal diseases that have a predictable length of time until death.

I agree that if behavior has changed "suddenly" your LO should be taken to Urgent Care or ER and checked for a UTI, dehydration, or other possible causes that would create the symptoms you describe.

I think the progression of the disease depends on what type of dementia it is and the individual who has it. My Aunt is 100-yrs old and has had dementia for at least 10 years. She has no other health problems but is barely mobile. She has fallen and broken bones 2x and recovered.

If it's a new or recently escalated delusional state, then I'd be calling for medical input asap. My grandmother lived with dementia for over ten years and was overall healthy until a series of events and infections in the last six months of her life.

Delusions are often caused by UTI's.

Of course, nobody can make a guess how long is left.