Replacing RO/DI filters

When is it time to replace the cartridges in an RO/DI unit? The cylinders are glass, so I can see the colors. And I think it's supposed to be send out a color signal or something??

I change my prefilters every couple of months, or when they look really dirty. Also, if I notice a reduced rate of flow, I change them

DI cartridges: Some are color changing, and will change color (mine last one went from purple to yellow)
If they're not color changing, you'll need a TDS meter. when the output gets about 4 I'd change it.

B

prefilter cart: every 3mo. (10micron)
carbon block: 6 mo.
RO cart: 1-1 1/2 yr.
DI: when exhausted(must test) or use color change.