As mentioned before, the RO water has a direct impact on the life on the DI. For calculations, please view the DI resin page on this FAQ. It also has an impact on a final filter's (aka - in-line carbon) lifespan. Since carbon filters are there normally to treat things like sediment and chlorine, the final filter has a good lifespan to begin with. And because RO water is free of those things, the final filter can last for at least 1-2 years. This holds true to any other post filter. Since the RO water is already very clean, post filters have a better lifespan than if you were to put them as stand alone filters or have no pre-filtration.