Let's see. Chemically, the white polystyrene foam underneath the shell will last forever, which is why it such an environmental nightmare. But, the polycarbonate shell to which the polystyrene foam is bonded has finite lifetime. After being exposed to UV rays and heat for a finite amount of time, the polycarbonate chemical and molecular bonds will degrade and lose their molecular structure. The polycarbonate shell then becomes brittle and stops being an impact spreading shell to the polystyrene foam underneath. I would like to refer to OSHA regulations (U.S. Occupational Health and Safety Agency) that require construction hard hats that have a thick polycarbonate shell be retired after two years.

So, two years would be prudent.

Mark