It sounds like your RO/DI system could use a general tune-up. First you need to understand the different parts of the system. There are four basic parts to your RO/DI system.
The first is the sediment filter. This removes particles from the water that might clog later stages.
The second part are your carbon blocks. These protect your expensive membrane by removing chlorine and chloramine (the former being more damaging than the latter), and other chemicals.
The third part is the RO membrane. This is where 95% of the work is done in the RO/DI system. Most membranes have rejection rates >= 95%. That means if the water coming out of your tap reads 250ppm TDS, it should be 12ppm TDS or lower after the RO membrane.
The final stage of the system is the DI resin. This removes ions that are difficult for the RO membrane to handle. After the DI stage, the TDS reading of your water should be 0ppm.
All of these parts of the system need to be serviced at different times, and you won't be able to tell they need to be serviced by looking at the TDS of the product water.
The sediment filter and carbon blocks are usually referred to as the "prefilters." In general, you need to replace these every 6 months regardless of how much water you make or the final TDS. It is possible to be smart about this and replace the prefilters only as necessary. You can replace the sediment filter only when water pressure before and after the prefilters drops. To do this, you will need a pressure gauge before and after the prefilters. If the pressure is dropping, the prefilter is getting clogged and needs to be replaced. You can replace the carbon blocks when they're exhausted by checking the waste water for chlorine or chloramines. If any chlorine or chloramines are detected, the carbon block(s) need to be replaced immediately. Chlorine will ruin an RO/DI membrane in very short order. Depending upon how much work you feel like putting into RO/DI maintenance, it might just be easier to replace the prefilters on a schedule, every 6 months. If you haven't replaced your prefilters in a while, you should do so regardless of the final TDS.
The RO membrane actually does not need to be replaced very frequently. My last membrane I had for over 10 years. You only need to replace the RO membrane when the rejection rate drops. To test this, you first must determine your membrane's rejection rate. Most membranes reject about 95% or more of the solids, so 95% is a decent rule of thumb. First, measure your tap water's TDS. Then, measure the TDS of the water after your RO membrane, but before the DI resin. Since the rejection rate of your membrane is 95%, that means about 5% of the solids should remain. So, take your tap TDS and multiply it by 0.05. If the reading after your RO membrane is greater than this number, you may need to replace your RO membrane. Keep in mind, however, that many things affect membrane performance, such as pressure and water temperature. This number may vary depending on your source water.
Because the DI resin is the last final polish, it needs to be replaced any time the TDS of the product water reads above 0ppm. If your system is running efficiently and you're staying on top of filter replacements, your DI resin should last a while. It's possible if your water has high CO2 that it might exhaust fast, but in general, DI resin should last a while.
