What are the realistic/optimal (or maximal) number of words of vocabulary that can be learned (retained and later successfully accessed) by an adult learner of a foreign/second language? (Unit: per day, week, or year.)
Read the Wikipedia article - result: Only focuses on children and their native language acquisition (as many articles do) vs. adults, who train conscientiously!
Google search: Which search terms would you recommend? Results from "vocabulary acquisition" and "amount of words/vocabulary" (+daily) are in review. I did not find much yet, see below..
Google Scholar: In progress... ("vocabulary acquisition" and "adult vocabulary acquisition speed")
Unverified statements found from Google search:
"I am a student at the Defense Language Institute. We are required to learn about 80 words a day." (Source) - I tried to access the DLI / DLIFLC website to verify this, but the website was down yesterday (31.08.2012, 1:30 am, GMT+9)