I've always been told (in bike shops) to replace a helmet every five years. However, when I contacted Louis Garneau to ask about a seven year old L.G. helmet, they told me it should be fine. So, don't know what the answer is. Probably at least every 10 years.
I was riding with my sister through a park once, when she skidded out on some mud and cracked her helmet. By pure coincidence, there was a walkathon going on for a brain injury research charity in the same park on the same day. On the way back, she handed them her helmet to use for publicity purposes.