every 2-3 years and everytime you have a bad fall. i'm not sure if they do it anywhere else but in Australia there are diffrent codes on helmets and some competitions they check your helmet to see if you have the correct code so if you have a helmet for 3 years or over you should probably see if you can get another one. i hoped that helped :)
Every 2-3 years, but if you knock your helmet you should get a new one even if the shell looks undamaged. The inside of your helmet has a shock absorption layer and if you knock your helmet, that layer gets condensed and stays that way (i.e., it no longer absorbs shock anymore). When you buy a helmet you should look into the helmet exchange program / crash replacement policy. For example, I have a GPA. I think they replace a knocked helmet within 12 months following my purchase for around $265 (even though the helmet is about $700 retail). Keep your receipt.
I would recommend changing your helmet as soon as it begins to show damage. A damaged helmet means it is less likely to work the way it is supposed to and do its job. Helmets should also be changed if they stop fitting you properly.
As soon as it shows damage you should change your helmet; damage means it is less likely to work the way it is supposed to and do its job. Helmets should also be changed if they stop fitting you properly.
Just a tip for when you are changing your helmet. Make sure that the helmet you are buying is up to the right standards. If you are unsure of which helmet to get, maybe ask your instructor for some suggestions. My equestrian friend bought a helmet which was not up to the right standards, and she had to return it (she got a full refund of course ) and had to return two helmets in total before she found the right one.