I find my DI
resin needs changing more often than the other filters. I change the DI
media when I start getting a non-zero reading on the TDS
Changing the RO
membrane would be when I start seeing more than 5% or so TDS
reading compared to input water, out of the RO
(but not DI
) membrane. I've never changed mine (less than a year).
In theory the RO
Membrane should be the least frequently changed item you have, DI
media probably the most frequently changed, and sediment and carbon block in the middle, though where these fall depend a lot on the quality of your input water.