3 years until replacement is nuts on a motorcycle helmet. Arai recommends 5 after it is used and 8 or so since it's manufacture.

On a bicycle helmet I'd actually think less time on a helmet because you sweat a lot more than on a motorcycle usually, and that's probably more of what destroys the helmet than the UV. The hard shell on the outside isn't going to get damaged to a point of not working because of UV.. but motorcycle helmets and bicycle are not built the same way so I have no idea whether a bike helmet should be replaced.