A Natural Solution Without The Serious Side Effects
These water jug filters aren't really rated for actual treatment of unsafe water, so I'm going to assume that you are using it for it's stated purpose of improving the taste. If that's the case, there are two criteria that should make it obvious it's time to replace it. The first one, the sniff test. Get some water out of an unfiltered faucet. Give it a sniff or a sip. Then do the same for the filtered water in the jug. If you can smell/taste the unfiltered water, but not the filtered water, you're good to go. The second is just flow rate. These filters will stop particulates, and if you're getting water from a well or spring, even a 'whole house' filter may let some fine particulates through. In this case, eventually the flow rate from the jug filter will start to diminish as it gets clogged. That's probably a good indication that the thing has reached the end of its useful lifespan as well.