As for general rule of thumb, sediment and carbon are 6 months, RO membrane is 2-5 years, and the DI is around 6 months to 1 year. As for me I don't have A Kent system, but a home water filtration RO unit with added on DI canisters. Your true way of measuring the filter depletions are with a TDS meter. Mainly because your local water is what determines how often you have to change the filters. The dirtier the tap water, the more ofter you have to change the filters. You'll have to measure the PPM's in and out of each canister, mainly the carbon and the DI, as the sediment will only change it slightly. Your major PPM reduction is after the TFC or RO membrane, and then a close to 0 or 0 PPM after your final filter stage. In my system, it came with 1 sediment, 2 carbons, and RO membrane. there is a final filter, but thats for the drinking water part. I then added tee fitting to a dual DI setup. This way I have pure water. One advantage behind dual canisters on the carbon and DI, is it allows you to measure the changes in PPM from the first filter to the second. When it the reading out of the first it almost no change in PPM, you no its depleted. I then place the second filter to the first housing and install a new one in the scond housing. This way you get full filter capacity, without sacrificing water quality or damage to the Rp membrane from clorine (this is in the carbon sections). DI filters the same thing. You can deplete the first filter and the second one will take up for what the first can't filter out whaen depleted. RO will start to produce less water as the clog. Flushing will help to extend the life. The man at <a href="http://www.aquaticreefsystems.com" target="_blank">www.aquaticreefsystems.com</a> was extremely helpful with the explantion of how to operate, check and maintain RO/DI systems. He can also tell you some tips on using an inline pressure gauge and pressure regulator to fine tune your RO/DI units. Anyway, just thought I mention what I found out so far about RO/DI units
