Hi Lois. It's really hard to say. Alot of it depends on his overall health but even then it's hard to determine.
I was told it could be anywhere from 1 yr. to 6 yrs. once the symptoms become obvious. 6 mos. to 4 yrs with other health issues.
But my mom had bronchitis
. The first medication they put her on caused a stroke
. Plus she had several more strokes afterwards. She lived over 5 yrs. But about a yr. before she died the only time she was coherent was when singing gospel music or when wanting to know why God wouldn't let her go home yet. It was heart wrenching because in those coherent moments she was aware of what was happening to her mentally.
But my great aunt had no other problems beyond arthritis
& she lived in that state for over 10 yrs. The last 2 or 3 of which she was already in another place. Her entire waking hrs. she'd just rock & stare into space.
There are medications now that can slow the progression. But it is painful to watch. Often personalities change and they can become combative. At that point it's more loving to place them in a home where they can get 24 hr. care. But visit often & take notes of anything that doesn't seem right. Also ask to see state investigation reports. By law they have to show it.
There are adult day cares that will pick him up & give him a few hrs. interaction with others while giving you a break. Most are free or on a sliding fee scale.
But your best bet is to call your state Long Term Care Ombudsman. They can answer most of your questions and/or refer you to those who can, as well as help you find others who can help.
But take care of yourself as well. I lost yrs. in physical appearance & made myself sick trying to take care of her. It's not the same as caring for an infant. This is a full grown adult with the mind of a child. Thus you have less control over their actions & words.
They can wound you deeply verbally but it's the disease, not their true feelings as many believe who haven't lived with it. My mom loved her grandson more than life itself from the day he was born but during an episode talked to him like a dog & threatened his life with an iron skillet. I thank God he was too young to remember.
Learn from my mistakes & get the help you need before it spiritually and/or physically drains you. I'll remember you in my prayers.