I just changed mine after 2 weeks and believe me ... it was dirty enough! LOL
One thing to consider is that BB will start to grow on the poly-fill as well so by changing them every two weeks, you avoid having too much BB grow on them and lessen the chances of a mini-cycle .. as remote as those chances are anyway.
I have it on the bottom tray so it captures the microstuff that gets by the filter pads which would gunk up my biomax rings. Leave the upper trays for biological media.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.