I keep mine until the pads are worn out (and starting to rub my skin raw), or the plastic head strap or mounts break. Oh, except for the helmet where the plastic outer shell blew off the foam innards flying down a long hill. May take three years, make take eight.

The shock absorbing foam itself? I've dissected several after replacing them because of an impact. Never saw any degradation. Of course the new MIPS foam may have better rotting properties (from the point of view of the marketers who want them to be replaced more frequently).