I did some investigating after you mentioned encountering racism and nuked a post containing a racial slur. Probably the same one you saw, so don't worry about trying to find it.
Oh, and to answer your question, the staff picks some new ones 'bout every 3-6 months or so, that is, if there's a need for some new ones of course. All based on their own merit-based system, and it's always worked well.