When is ideal to change RO cartridges?

[Opus]

Well I think it was time to change the sediment and carbon filters. I took off the RO and DI and used the flush caps and came out with a higher tds. It was 34 in and 46 out. Changed them out and then came up with 34 in and 29 out. Put the RO and DI back on and it was at 0 again on the final out. Looked at my DI filter and it has just started to change at the top of the cartridge, so that and the RO may be next on my to buy list. Not bad considering this is the 3rd time changing the first 2 cartridges and 3 years on the RO and DI cartridges.

The RO guru that used to post on here had tds over 500 and he had 8 years on his RO membrane and was still getting 99% rejection rate. With your low tds you should get 7 or 8 years as long as you keep changing out the pre-filters. My guess is you should be able to get 4000 to 5000 gallons of 0 tds from the DI filter before it needs to be changed.