The Carbon Filter is not the one to skimp on. The relative cost of the carbon filter is very low compared to replacing your RO membrane because of Chlorine damage.
I see customers suffer from shot RO Membranes due to lack of carbon filter changes every month.
Replacement timing Varies by Machine and Manufacturer and has to due with the number of gallons the Carbon filter can treat. The low is 2000 gallons and the high is 80,000 gallons.
Some manufactures “kit” the carbon filter with the DI.
For instance, for the HydroCart and the HydroTube, IPC Eagle sells a kit with the Carbon and DI Cartridge together that way they feel you are always covered.
This does not work for systems with large carbon filters. The amount of Carbon in an EZ pure will last the typical window cleaner working 6 hours a day 5 days a week with one pole 8 or 9 months, yet the suggested replacement interval is 6 months. I expect that this is to be able to cover those window cleaners that that use 2 poles or work 6 days a week.
Unfortunately, there is no simple chlorine (inexpensive) tester to see how your carbon filter is managing to filter the city water, so I recommend that you change the filter on the recommended schedule for your unit.