My father's decline from early onset dementia (Lewy body variant) was fairly rapid. He was in a nursing home the last six months of his life, and his utter confusion and helplessness were palpable. He lost the ability to walk, feed himself and converse. He rarely seemed to recognize his wife of 45 years, my mother, but one time looked right at her, called her "sweetie" and told her he loved her. He died a month or two later.
Incidentally, I see many similarities between his behavior and that of our ersatz president.