How long do helmets last?
Five to eight years seems reasonable.  See here: http://www.helmets.org/replace.htm - some helmet makers recommend somewhere in this range, and longer than eight years you start to see deterioration from sun damage, occasional knocks, etc.

My last helmet lasted only four years in high-UV riding, about 15,000 km total, before the plastic strap around the back simply cracked in half.  This didn't make it 100% useless or ineffective, but it surely prompted replacement.  But this was a lot of riding, mostly in bright sun and high temperatures.