How long to people diagnosed with benson's disorder generally live and in what condition? About how many people are believed to have been diagnosed?

Few yrs, rare. Benson's syndrome (also posterior-back brain-cortical atrophy) is a rare neurodegenerative disorder start in vision cnter with inability to recognize familiar objects or people. It is a syndrome, because it can be part of another disease, such as alzheimers or jacob-creutzfeld). Mild to severe, 1 to >10 yrs, stable for yrs or progressive disability. First 2 yrs at specialized center is crucial.
Not very much known. Benson's disorder (posterior cortical atrophy) is believed to be a visually impairing variant of alzheimer's and is very rare. See this landmark paper from 2004: http://www.Orpha.Net/data/patho/gb/uk-benson.Pdf a consult with a neuro-ophthalmologist may be helpful.