Old Bell ad:
"If you've got a $10 head, buy a $10 helmet"
not verbatim!
General rule of thumb is every 3 - 5 years depending on use.
I live in Florida, and if you leave a helmet in your car for extended periods of time, count on 3 year cycle. I don't know about you guys, but I do fall when I'm mtn biking. Most of the time there is no noticeable damage to my helmet, but who wants to take that chance? Not me, I'll spend $75 every 3 -5 years for a new one!
L8R
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger