A helmet protects only while its shell and suspension are in good shape. Replace after any impact, and check regularly.
After any impact, take the helmet out of service — even if the damage is hard to see. A shell that has absorbed a blow may not protect the same way again.
Check the shell and suspension often: cracks, dents, fading, brittle parts, frayed straps or a loose fit are all reasons to replace. Keep helmets out of direct sun and heat when stored.
Ask the supplier about the replacement guidance for the style you buy, and keep a spare stock so a damaged helmet is swapped the same day, not next week.