I haven't been able to find an answer when a particular helmet needs to be recertified/reconditioned, but here's an article about it. Recertification doesn't mean just certifying helmets that had been decertified (like the R), but helmets that are older, like the CPX-R or the Pro7. Maybe reconditioned is a better word. The second article below talks about the reconditioning process for helmets (football) and limitations on it.