Unless you have high sediment or high chlorine, the prefilter and carbon block should last 6 months. DI cartridges are entirely dependent on how well your RO membrane is functioning and how much water you have made.
Prefilters and carbons have very little to no effect at all on final TDS, they are there to protect the RO membrane.
Again, if you have a RO/DI you need 3 TDS numbers to determine what you need to replace.
1. Tap water TDS- not as estimate or approximation but the real deal
2. RO only TDS before or without DI- If you don't have a DI bypass then either remove the DI cartridge, run some water to fill the empty canister and take a reading or disconnect the 1/4" tubing from the RO membrane down to the DI canister and take a reading there. This is the most critical piece of information you need.
3. Finally take the RO/DI TDS.
If you are using a glass, make sure it is absolutely squeaky clean with no soap residue, water spots or film. I use a glass dedicated for just this purpose so it stays clean. First test the tap water TDS, triple rinse the glass and meter in DI water. Then test the RO only TDS and again triple rinse the glass and TDS meter in DI water. Finally test the RO/DI TDS and do a final triple DI rinse and put the glass away dry and upside down to keep dust and contaminants out of it. Always keep the cap on the TDS meter and remember to triple rinse with DI after every use so you don't ruin the probe.
Chances are you need a DI cartridge but without knowing how well the membrane is working its a guess. If the membrane is functioning correctly you should be seeing an RO only TDS in the 2 to 4 range. If that's the case you are adding contaminants back into you water due to exhausted DI resin and that's not good. The weakly ionized substances will release first and those include phosphates and silicates among others and you don't want those.
I would not replace anything until you get good reliable TDS readings from all three places to tell you what you need. No need in throwing money at it when it may not be the problem.