First of all, there are two kinds of DI filter cartridges; vertical and in-line. Both can filter out the TDS down to zero (or very close to it), but they have different filtering capacities. To figure how long they'll last is simple math. Take the capacity of the DI filter and divide it by the TDS of the water feeding it. The lower the TDS feeding it, the longer the DI will last. This is why it's good to have a RO membrane to remove most of the TDS before using a DI. For Example: The vertical DI has a capacity of 3,000 ppm. The in-line DI has about half that capacity, so around 1,500. Let's say that the RO is reducing the TDS down to 10. This is what the equation will look like for each DI: Vertical DI: 3,000 (ppm) divided by 10 (ppm) = 300 (gallons of pure [-0- TDS] water) In-line DI: 1,500 (ppm) divided by 10 (ppm) = 150 (gallons of pure [-0- TDS] water) There are some exceptions to this equation. Higher than usual levels of some TDS can cause the DI to exhaust faster. It's also important to know when to flush the membrane before starting to make RODI water. Some examples of elements that, if in high levels, can cause the DI to exhaust faster are: phosphates, nitrates, silicates, salt, calcium, etc. The RO membrane can remove most of it, but the DI would have to work harder than usual to remove it all from the water. If the unit has sat inactive for more than one day and you do not flush off the membrane before making RODI water, it can cause the DI to exhaust faster as well. Make sure to flush off the membrane for one minute per day of inactivity. This doesn't really apply to those who use their unit every other day or every day. However, if you use it that much, you should still flush the membrane off once a month for about 30 minutes or so. Instructions on how to flush are in the Fittings FAQ.