[crazygringo]

Site creator here, again -- my research (small sample size) having Brazilians take the test is that a couple years of learning will get you around 1,500-3,000 words, several years around 4,000-6,000, and a good student with, say, 8 years of classes might get around 10,000.

Beyond this, it's pretty much necessary to live abroad for an extended period of time, or be exceptionally good (driven) at languages and watch TONS of TV, use tons of online chat, etc.

[monopede]

English is my second language and I got ~13,500 words. I was surprised that this was only half the average. I read a lot of scientific papers in my field and almost never come across a word I don't know. Writing technical papers well also requires not much vocabulary (it's more about clarity of expression). I believe to get beyond that number you simply have to read lots of books. I notice lots of words I've never heard of when reading, say, Bill Bryson but I very rarely bother to look them up because they usually make enough sense from the context.