Cartridges can last up to about 5 years depending on how often they are cleaned and how heavy a load they have to filter...In some applications they might only last a year or two (think biguinide). If they are not getting white it might be organics impregnated in the filter material and trapping other dirt also. This is why it is good to soak them yearly. TSP is good at getting rid of the organics....Sometimes a soaking in a bleach solution works also (anywhere from a 10:1 to a 4:1 dilution of regular bleach). If this doesn't help then it's probably time to replace the cartridge.