It should never need replacing, but will need occasional maintenance.
In an ideal world, you'd have sufficient mechanical filtration prior to the Cirax so that the Cirax does not get covered in particulate matter. However, even the most stringent mechanical filtration will let small particles through, and over time you may find that the cirax gets covered in sediment. This may reduce it's effectiveness, so you can give it a gentle clean every now and then.
I would recommend that you split your cirax into two or three or more media bags/boxes and gently clean the different portions at different times. So, if you have two bags/boxes of Cirax then you can clean each one alternately every 6 months. The more portions you have, the more stringently you can clean (or replace if you feel like it) each one (alternately, not all at once) without so much of a worry that your overall biofiltration will be significantly reduced. Put the cleaned portion back in as the last filtration stage.
Try and make sure the mechanical filtration has a very fine final sponge/pad, although this may mean that you have to clean/replace the final stage sponge/pad more regularly, the cirax will need less maintenance.
