I don't think the helmet manufacturers would state a definitive life span... the legal possibilities there could be bad... For some reason, I have a bunch of helmets... 3 or 4 normal types... two Vigor full-face... and my old Bell FF MX lid that suffered through my early DH racing days (quite scarred, visor broken off...).
I tend to have trouble finding a helmet that fits well, so when I'm in the mood to try them on, if I find one that fits I buy it... regardless if I really need it or not... I have a similar issue with shoes... I have 2 or 3 pairs of Nike Cairnes in the closet, NIB, that I ordered when they wre on close-out years ago. IIRC, there's a NIB helmet in there as well.