You should change 2 of the filters every six months. The DI resin should be replaced as needed (approx. 8 months depending on how bad your tap water is). Depending on how much water you make, the RO membrane should last 2-3 years.
Dual in line TDS meters are the way to go. This way you can check the TDS of the water coming out of the RO membrane and the water coming out of the DI. This will let you know when to change the RO membrane and the DI resin....