Do you just have normal 10'' housings?(i cant tell if the prefilter you put up is normal size)
Normally prefiters should be changed around 3-6 months but it says you get 4x the capacity, so i take that to mean it should last 4?x longer?(alex should be able to tell you if you send him an email)
I get my pre filters from collins water products, i think you get 6/7 sets forbaround £60/£70.... (Google)