Product review (5 stars): Perfect Replacement
Great that this is a two pack.  They suggest replacing every couple months.  I just let one go through the heating season.  I do not have hard water.  When I check it, if it's a little dirty, I place in flat pan with water and lime away and soak for about 15 minutes before rinsing.  I then let dry for a day.  It may make it another season after that.  Now I don't have to worry.