There are several factors that can affect the life of the filters. It also depends on what filter you are talking about. The membrane lasts quite a while. The carbon and sediment filters will also last a while on their own. They have different ratings depending on the cartridges you have. The DI will exhaust the quickest depending on how clean the water you are supplying to it is. Flushing the membrane regularly and running the filter for longer durations will help extend the life of the DI resin. It's nice to have a dual TDS meter so you can see the readings on what's coming out of your RO and what's coming out of your DI. You'll be surprised how high the TDS gets just sitting between the RO and DI stages (i.e., TDS creep).