Test by pouring water over the surface of your car. See how well the water beads off. With a fresh coat of CQuartz or Opticoat the water should pretty much completely bead off. No water should stick to the surface of the paint. If it does reapply another coat. I say 18 months is the maximum due to the nature of hands washes and weather that erode the coat over time.
Or you could just do like me and forget the Cquartz/Opti-Coat all together and just wrap your entire car. The vinyl protects much better than anything else.
