I'd love to see a study that examined six month+ retention on amphetamines. Part of the reason I'd love to see that study is that it would have to examine six month+ retention in a control group, too. I suspect we'd all be pretty disappointed by the results. We all "ha ha" about people cramming for the test and then immediately forgetting it all, but I think that we all know that that is actually the normal thing for most people. Putting numbers to that would probably be pretty sobering.
(We've talked a lot on HN about whether it's worth it to go to college. That wouldn't be a half-bad metric to use as part of that decision; do you actually recall stuff from more than three months ago in school? If not, don't spend money on college. Obviously, this is not necessary to live a good life because most people seem to manage without it.)
I have to admit I don't remember a lot of the organic chemistry textbook and AD&D Player's Handbook I inhaled when I was 9 and on Ritalin, but I apparently remembered enough of the book on phone switches that digital logic was always a cinch. But that was 23 years ago. I think even five or ten years later, I remembered quite a lot of it.
However, as I commented below, I am skeptical as to how concrete the amphetamine driven associations persist 3-6 months down the line.