Its the one subject I am pretty knowledgable about!

The RO membrane will go on being efficient until it fails. You will either see a sharp increase in TDS meaning the TFC fabric has ruptured due to too much solids build up causing it to find a wek spot and tear, or you will see a decrease in GPD which is often the case or possibly a combination of both. Again the sediment and carbon have little to do with TDS so you monitor their conditions using a pressure gauge or gauges before and after such as I do and a low range chlorine test kit. TDS readings don't tell you anything about their condition.