In my setup I use three filters, a 10 micron spindown sediment trap, a charcoal canister and an Everpure ES07 (softener and filter) to get all the water numbers in correct alignment for drinking and machine care. I use a TDS meter and test strips. I know when it is time (besides logging dates) because you can tell when the water starts to taste differently, and therefore the espresso taste starts to go off.
