Basically there are 3 stages of filtration on a RO/DI unit, the prefilters, the RO membrane, and the DI and it can be difficult to determine when to change what. Here's what I gathered from my research back in the day.
The prefilters should be changed out every 6 months to 12 months, depending on your water quality. These need to be changed on a timeline regardless of what your TDS is reading, because dirty prefilters allow larger particles through, meaning your RO membrane has to work overtime, exhausting it much earlier than it should be. RO membranes are expensive. Prefilters cost $20 for a set.
The RO membrane should remove 95%-98% of the TDS from your water source. To determine whether or not your membrane needs replacing, test the water right after it comes out of the membrane, but BEFORE it goes into the DI cartridges. Then, test your tap water. Divide the after-RO TDS by the tap water TDS. This should only be about 3%-5%. If it's any more, you should consider replacing your membrane. Bear in mind, to my knowledge this reduction is proportional.. if you have tap water at 25 TDS, after your RO membrane you should get around 2 TDS. If your tap water is around 400 TDS, you might be getting as high as 20 TDS after your RO membrane.. it's still the same 5%, it just looks a lot larger and will be harder on your DI. That's how the tech works, from my understanding. As long as it's around 5% or lower of your tap water, your membrane is fine.
If the water after the DI is above 0, your DI needs replacing, whether your membrane is fouled or not. I would also check your RO membrane output at this time to ensure that your membrane is still in good working order, but that's the general schedule you should follow.
