My Father Has Alzheimer's Disease. In The Past 2 Months

My father has Alzheimer's disease. in the past 2 months we have seen major changes. His walk is a shuffle and abnormal. He is rooked and unable to put his head all the way back. a brain scan was just done because his dr. thought he might have Parkinson's as well. however, it relieved that his brain has atrophied quite a bit. I was wondering what the life expectancy for this is.