The sediment filter will stop production if clogged with silt or other so unless this happens don't worry too much about it, Iv let my carbon filters go for a year before changing with no problems, as I always say, smell your water for chlorine if there's enough chlorine in it to damage the membrane you will definitely smell it, pre filter sellers don't like to let on about this but chlorine in small amounts wont damage the membrane, if it did there's no way a few quid's worth of carbon would protect it.