Well, my grandmother had dementia.

Her illness progressed very quickly in the sense that it only took her about a year for her to regress into an infant mentally. She spent about 10 - 13 years in bed screaming like a child.

My point being that dementia can progress quickly, but you can still live with it for a long time (if you are unlucky enough). We, as in her family, spent years hoping she would die, while she spent years suffering.