When Do You Change Your DI Resin?

[rastaangel]

So I was just wondering at what TDS reading do you change out your DI resins? I have a lack of storage space in my place so I use the same container to fill my bottles with RO/DI water as I do to mix my new salt water so I never get 0 TDS in the bottles anyways.... So I usually change mine out when the water coming out of the RO membrane is the same as the water coming out of the dual DI chambers, when the DI resin is totally exhausted which gives me a final TDS reading of 4ppm