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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.