Used to work in a tack shop and the manufacturers said you should replace if you have a fall and knock it or if you drop it on the floor! Or after every few years. They really are quite fragile.
They work by the polystyrene inside absorbing any impact. If you fall, or drop it from any height it dents the polystyrene thus making it less relyable next time. I try to replace it after every heavy fall which is why I have a posh hat for posh occasions and a cheap hat for everything else!
My current hat is a new one - well about 9 months old. Only bought it as the lining of my other one went all bitty so as well as having hat hair after a good ride I had bits in it too *gross*
This is what we have been recommended by a hat manufacturer: A helmet is lined with microscopic bubble wrap. The bursting of the bubbles absorbs the impact. The hard shell spreads the area of contact over a much larger load bearing area. Helmets should be replaced after a blow to the head, especially if the wearer loses consciousness. The expected life span of a helmet for the average rider is five years
If only I could find a hat to fit my head I would change it a bit more frequently than 10 years! I have a stupid big head (full of brains - not!!) Next option is hand made for me, better start saving!
I've just changed mine. Fell off a couple of weeks back and landed against the wall. There was a massive gash on my helmet and slight creaking sounds made me think it was time for a new one! I had my hat for about a year up to this point although I read somewhere that hats need replacing every 4-5 years if you don't drop them or fall off in the meantime.