How long should an academic CV be for someone applying to post-doc positions in Europe?
The notion of a curriculum vitae is confused by the fact that it gets used in two very different ways:


A comprehensive list of all of one's academic accomplishments
A short summary of one's background and skills (like an American resume)


For a postdoc position, you will generally want to be using the first---at least, that has been the way it has been for all countries that I am familiar with. 

That, then, may be as long as it takes to list everything out; when applying for a postdoc position, your record will likely still be fairly short.  Well established researchers, on the other hand, can easily end up with 20+ pages and may consider switching to a "best of" format in which only the most significant highlights are shown.