most every creatine supplement i've tried says "use it for about twelve weeks, then take a couple weeks off," which may or may not have anything to do with the lack of long-term research.

typically, i will use it for as long as a bottle lasts, with the occasional days when i forget to take it altogether (like, er, today... but i had a steak for dinner, so there's creatine in that), and then ignore it for a couple months.