its not your water causing this, as you state water out is 000tds. with water in at 66ppm it will last a fair time. my water in is 105ppm & my 10ltr resin canister lasts approx 6-8 weeks (filled 3/4 full). I use 2 canisters in sequence though (for resin economy). when i only used 1 canister i would get about 4 weeks. i use wfp nearly every day & run a 400ltr van system. my very first clean with wfp dried all white on the glass, i put this down to all the hoses beeing new & maybe it was flushing the crap out of them. after a couple of windows it was fine.
it is probably more likely your technique that is the trouble, with being new to wfp, me personaly i never clean the top frame of a window for the reasons your experiencing.
hope this helps
tony