The filters life depends on your water quality. You should be measuring your water out of the Filter unit with a TDS meter and it should read 0 with new filters. I replace my filters when the TDS reads more then 4 or the DI resin is 80% Orange. When we were having all the flooding in spring my filters were lasting a couple months due to the high bacteria levels in the City water but normally they last 4-6 months. These are high quality filters, not the cheap stuff that is repackaged by some of the Aquarium companys. If you go to Lowes and look for a 5 micron sedimet filter you'll pay $10 for 1. I like having them on hand for 2 reasons.
1. They are very expensive to buy. these filters probably cost around $35.00 (for the set of 3) if you can get them this cheap. Online about $25.50 + $7.00 shipping
2. Only 1 store in town that carries them.
Keep in mind this does not include the DI filter which usually is about $25.00 or more.
I am considering ordering some DI Resin but at a later date.
If you guys want to go in on some DI Resin we can get 44 LBS of it for about $229.00 plus shipping, That will fill about 35 cartridges (you have to have refillable cartridges but I have never seen a DI cartridge that wasnt refillable). If you divide the price by 35 its about $6.50 a pop which is great compared to over $20
If you guys want to go in on the DI Resin Ill take 5 of the refills which means we need 30 more