Mine is usually around 50,000 posts, but I delete almost everything after six months, and some forums get cycled every two weeks. That was originally to keep our web host happy, but I took a vote when we moved to our own server, and the users liked having threads deleted regularly. So we still do it.
I wind up deleting about 500 posts every Friday on the short-term boards, and then another 5,000 at the beginning of each month. A few threads get marked for archiving, but I'm pretty picky about what we keep.
