If you mean a system cleaner like CCleaner, I would say every few months. No reason to get all OCD about some temp files and whatnot, although I generally do run CCleaner after a new build and install of all updates and programs.
If you mean an antivirus, it really depends on your surfing habits but in any case I would recommend running a real-time antivirus to check incoming files. I run a full disk antivirus check monthly, but then I only go safe places and use hosts file blocking to avoid bad places.