There was a site I read that recommended once every 3 years, but then another said once every 5 years, then another said 10 years, so I think it just depends on the level of degradation that is going on with the helmet. Like I said about my own personal ones I replace when the lining or the foam starts to deteriorate which ever comes first and lately that's been the lining material. This of course does not include impacting the helmet, if you have impacted the helmet it's probably wise to replace it even though you don't see any damage, foam does crush and crack and sometimes those issues can hide from the eye of inspection.

I think the 3 year replacement camp is probably manufacture driven, they want you spend your money. If the helmet is lasting longer without the foam or lining degrading then keep wearing it...I would keep wearing mine forever if one lasted that long.