Dad was recently diagnosed with dementia. Last summer he was able to pay his own bills, comprehended probably 99% of every conversation, good appetite, worked in his yard. Now just over four months later, he barely eats, wants to sleep all the time, days go by with him talking to himself as though he were at work, he sees people who aren't there that no one every knew, and he sees family members who have passed, but he still knows our immediate family when he sees us, but gets our names mixed up, he's always done that though. Is it common for dementia to get this bad this fast? I've heard it takes years and years for it to progress.
So sad.
Is Dad seeing a dementia specialist, such as a geriatric psychiatrist or a behavioral neurologist? Few internists or general practitioners are knowledgeable about the care of dementia, but specialists can often develop a treatment plan that will help manage the symptoms.
My husband had Lewy Body Dementia, which is often said to last 5 to 7 years. I belong to caregivers support group for this disease. One member's loved one declined extremely rapidly and died in less than 2 years. My husband lived 10 years, all of it at home. Another member's husband lived 12 years, but several years in a care center. The other members loved ones fall somewhere in-between. In other words, you may be able to find averages and general statements but it really depends on the TYPE of dementia and then within that the INDIVIDUAL.
Here is something else to consider: My husband entered dementia at the deep end, having very severe symptoms to start with. With a treatment plan from an expert and also with the passing of time, his symptoms actually decreased. The expert later thought it might have been an inflammation in the brain, trying to fight off the "foreign bodies" of the dementia.
My advice to you is to first be sure Dad is being followed by an expert in his disease, to try what the expert suggests, but also be prepared for whatever the disease throws at you. It is a sad disease, with no cure as yet.