Product review (5 stars): my filter looks almost like new. Make sure to soak the filter in ...
I use these filters regularly in my evaporative humidifier.  Two things makes them last much longer:  (1) buy the water treatment cartridge that is also sold by Protec and put it in the water tank.  Just doing that doubles the time between cleanings.  (2) When you clean your humidifier, take the filter out and soak it in a small container filled with white vinegar.  The white vinegar disinfects the filter, and it dissolves mineral deposits.  After 2 hours in vinegar, my filter looks almost like new.  Make sure to soak the filter in clean water and rinse well before using so that you don't spew vinegar into the air!