IIRC 15k-20k is usual for monolingual adult speakers of English. Since different languages have different total vocabulary size -- English's is particularly high -- it probably varies quite a bit across languages. I believe bilinguals tend to have a smaller vocabulary in each language, but larger if both are counted.

You can estimate your English vocabulary at http://testyourvocab.com/ (BTW, if anybody knows a site that does a similar test for other languages -- particularly German -- I'd be interested).