I've read that the microns and carbon blocks should be changed about every 6 months on an RODI. also the RO should last a couple of years under normal conditions. how long should a DI last before being replaced?
It really depends on the quality of the water you are using. Here in Philadelphia where the tap water is horrid a DI cartridge might last six months if you're lucky, but in a more rural or mountainous area the tap water quality can be quite good and a DI cartridge may last two years. The easiest and cheapest way is to test your RO/DI water once a month or so with a TDS meter. When it no longer reads zero, replace everything except for the RO membrane.
According to a popular manufacturer's site, a full size DI cartridge should process 6,800 ppm. Divide your RO output into 6800 for how many gallons until replacement. i.e. if your RO output is 20 ppm, 6800 divided by 20 equals 340 gallons. An inline slim DI will process half of what a full size will. I do not replace everything when the tds goes up. I just replace the DI and my tds goes back to 0. I replace the rest of the filters every other time.