A great question. And the answer is...it depends. (We're lawyers, right?) But I found an IBJ article by Anton Mikel from 2004 that describes in detail which kinds of files have to be kept for how long -- to the extent that's determinable -- and who owns the contents of a client's file (is it the client or the lawyer?). The article is here. And I found it listed among many promising entries on the "File Retention and Management" page of ISBA's Practice Resource Center. The Center is a practice resource in itself, a place "where we've assembled articles, ethics opinions, and other tools to help inform your decisions about practice-related issues." Check it out if you haven't already. I'll point to it from time to time to remind you it's there.