Product review (5 stars): Every six months!  Clean water!
I'll point out that you can save some money with these filters.  There are about half a dozen iterations of the exact same filter, but a different part number.  It can be under Aqua Fresh or Kenmore or Whirlpool.  So search around.  I've tried all different kinds and they all work fine.  Nothing but what's cheapest for my starting-to-show-its-age 9-year old Kenmore Refrigerator.