quality > quantity
I doubt there are 400,000 papers worth reading in all of medicine. A major skill in research is understanding what to read, and how carefully.
Prepping to write a review usually involves about 150 articles, for me. About 50 of them just need a cursory look, and the abstract often suffices. Another 80 or so need some more attention; you might be looking for patterns in the methods used, cross-referencing results, figuring out what labs are doing what, tracking authors over time, etc. This is how you understand the state of a field.
The last 20 or so are the bulk of your time. A few are genuinely excellent and will be a highlight of the review. Some are simply solid work that requires a good amount of attention. A fair number are terrible and highly misleading. These are the papers that look okay, but have some flaws that need to be addressed by the field at large. It takes some tact to write about these, but it's usually not too bad.
Finding good authors, especially ones that write good reviews, can get you up to speed in a field in an afternoon. An evening chasing down citations and digging a bit on your own, and you're probably about where Bill was with this. Since Alzheimer's is a pretty big subject requiring familiarity with a variety of topics, I'd give it two weeks.