I was told on my CBT (UK motorcycle training course) that you should look at replacing a thermoplastic helmet after 3 years and a fibreglass one after 5.
Seems a little too cautious to me though - there are thermoplastic helmet manufacturers that offer 5 year warranties; there's no reason why they should do that if the helmet would be past its useful lifespan in just 3.
Does anyone know how long this deterioration actually takes a) in storage b) in use?
I agree that would be overly cautious on the plastic front, but I would replace any helmet after 5 years due to the inevitable compression that occurs with the energy absorbing foam over the years. In addition, after 5 years of use, a helmet gets bumped around a lot and really is due for it anyway.
Giving different recommendations for plastic vs. fiberglass is pretty suspect, in my book. The shell is seldom the problem or the reason to replace the helmet. It's the energy absorbing foam and the glues that degrade, not the shell (unless you leave it sitting out in the sun all the time - and by all the time, I mean way more than just during a ride). The shell is protected by paint and is good for longer than the protective materials. If plastic degraded that quickly, we'd be in real trouble with all those body panels on our bikes.
As for the original topic, I wouldn't buy a Snell certified helmet myself as I think I paid far too much for my current helmet as it is. There are perfectly good and just as safe (SHARP 4* or above) helmets at quite a bit less than I paid for mine (I've noticed the cheapest, made in China helmets are usually subjected to more tests than others - making one helmet suitable for all markets. Don't know about comfort though). I'd be absolutely livid if I dropped it and had to replace it before its expected lifespan. Obviously being in the US though you'll have to make sure its been put through the DOT tests as well as the ECE & SHARP tests.
Several helmets that meet both Snell and DOT standards are well under $150. You're not going to save much more than that and still get a helmet that either a) is comfortable to wear, b) doesn't deafen you from wind noise, c) isn't a pain to remove the shield, padding or other things you might want to swap out or clean or d) doesn't fall apart after a year or so.
My HJC was around $130, is relatively quiet, is comfortable, offers good venting, is solidly built and removing the shield and head liner can be done in seconds.