i change out small amounts more often as oppsosed to larger amounts less often , as per what RHF recommends and has always worked great for me
i dont measure or weigh it out but i run maybe 1-2" in a tlf 150 reactor
i change mine out maybe every 2-4 weeks , i dont change it out so much based on the P04 test numbers but i can visually tell when my gfo is exhausted