What is the average amount of new vocabulary adults retain when learning a second language? 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.)

Initial research:


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)
Try keywords like: "second language learning number of words per"

Some suggest 3-6 words per hour of reading, which is afterwards used for calculation of average per week, month, year. References of studies to support this claim are missing though: 
http://ip-173-201-189-182.ip.secureserver.net/archives/sla/waring120304.pdf

Several times I found references on this study: Miltion and Meara (1995). 


  A longitudinal study by Milton and Meara (1995) found that adult
  learners of English as a second language could learn 2650 base words
  per year. more at:
  http://iteslj.org/Articles/Cervatiuc-VocabularyAcquisition.html


I remember it should be 7 words a day. If it is not, it should be at least divided into blocks of this size. If you google this number, it is mentioned on several sites, I do not remember the book I got it from.

I would be careful using these numbers, it is likely that they depend on the techniques that students and teachers tend to use.