For rock climbing helmets it used to be suggested to replace every 3-7 years depending on the amount of time spent outside. Supposedly, materials used in helmets begin to breakdown as soon as they are done being manufactured. However, a lot of testing was done a few years ago and the results were to basically go with your intuition on when to retire it. I plan to replace my helmets about every 10 years whether it's for bikes, climbing or motorcycles. My head is is worth at least that.