When Do You Change Your DI Resin?

[rastaangel]

I am using a dual inline meter. Its checked after the RO membrane, which usually is 4 ppm. Then again after the DI resin which at this point in time reads 0 to 1 ppm.
I was just wondering when other change there resin, as I usually dont change it until it reads the same after DI as it does before.
Both of my DI chambers are blue so I cant see what color it is