How do you know if you need to replace the RO membrane filter in the culligan good water machine

When the TDS level jumps above 50 ppm or when the TDS reduction is less than 90%, change the membrane.

A TDS (total dissolved solids)meter is needed to test the Reverse Osmosis water and compare it to the feed water to determine how much TDS is being removed. Reverse osmosis membranes are designed to remove 80-95% of the TDS in your water supply.

For example: If your water has 500ppm of TDS the minimum TDS removal should be 400ppm leaving 100ppm. If the TDS from the Reverse Osmosis System drops to these levels it is time to change the Membrane.
Typically an RO system will consistently remove 90+% of TDS for many years. The normal lifespan for a membrane can be anywhere from 3-10 years. It all depends on the TDS of your water and your water usage. The higher the TDS the more frequently the membrane needs replacing. The other filters should be replaced every 6 months to keep the quality up and protect the membrane.
A TDS of 1000ppm compared to 100ppm is much more difficult remove to acceptable levels. I like to compare 1000TDS to taking muddy water, forcing it through a filter and getting drinking water. Where 100TDS may be more like a clear glass of water. Consequently a reverse osmosis system on high TDS will be less efficient, less gallons produced and filters will need to be changed more frequently to keep the quality and quantity up.
RJ