I change it every 3 years or every time I hit my head in a fall. The integrity and safety of the helmet can be compromised every time there is an impact so it’s important you avoid any impact and replace it after a fall.
Every time you fall on your head. It looses all its strength with the first impact.
i like changing mine every 4 - 5 years :)
I change mine after each big fall (fortunately it doesn't happen too much), even if the helmet doesn't seem damaged, it can be spotted inside so careful.
I change my helmet every 3 or 4 years. My parents are really concerned about safety and prefer when I ride with relatively new equipment :)
I like to change it every 5 or 6 years, even if I don't fall a lot as the inner foams can get flatten over the years.