@anamur-- Yes, I have heard of ashwagandha being used for Alzheimer's disease. There are various studies that have been done on ashwagandha extract that found it very beneficial for mental cognition and memory. Since Alzheimer's affects these abilities, ashwagandha can be beneficial in treating and delaying these symptoms. Of course, no one can guarantee results.
I don't know about the amount that should be taken but the recommended dosage should be applicable in this case too. Always check with your doctor of course.