i think that is totally depends on how much you ride and how often you fall. i change my helmet every 2 years but i ride almost everyday and show twice a month
Everytime you fall or if it's five years old. I never had a helmet that lasted that long with the horses I ride though😂😂
Every 5 years or if you crack it when you fall off
It is recommended to change your helmet every 5 years because the foam lining you have inside get damaged even if you never fell on your head with it (on some brands you can buy a new foam lining).
Moreover your helmet might be damaged even if you can't see with your naked eye :)