You are supposed to check the phosphate level in your tank and also at the outlet of the reactor. If you levels are 0, you don't need to change it. If you get a reading above 0, you are supposed to slow the flow through the reactor, then check again in an hour. If it's 0, don't change it, but, if you do get a reading at the output of the reactor, you change it. I have seen people not change it for months with 0 phosphate readings, and, when they finally do go to change it, it's hardened to rock-like consistency, so, I'd say, it should be changed more frequently than that. I found all that info on another thread here.