This is an excellent question and one that brings up a number of different answers, depending on the types of hardhats we're dealing with.
In terms of due diligence, the best answer I can give is to simply follow the manufacturer's instructions. They will cover what you need to know about replacing the shell and suspension, as well as storing, inspecting, cleaning, and maintaining the hardhat.
It's important to also consult the relevant standards. For industrial head protection, consult the ANSI/ISEA Z89.1-2014 (R2019) American National Standard For Industrial Head Protection. If your jobsite is in Canada, also reference the CSA Z94.1-15 Industrial Head Protection Standard.
You will probably need to replace the hardhat's suspension more frequently than the shell itself. One manufacturer, for instance, recommends replacing the suspension within 12 months of use while the shell itself can last up to five years without needing replacement.
(Learn more in Bump Caps vs. Hard Hats: Which Is the Right Head Protection for the Job
?)
Look for Signs of Wear and Damage
The shell and suspension should be replaced sooner, however, if you discover any defects or damage. Workers should inspect the shell daily and look for signs like fading, crazing, dents, scrapes, holes, or cracks. When checking over the suspension, look for tears and fraying.
If a hardhat is damaged or starting to wear in any way, it should retired from use right away and replaced.
Pay Attention to the Date of Manufacture
Companies that buy hardhats in bulk will issue them from storage. Sometimes, those hardhats have been kept stored away for years before a single use.
Although they look new, it's important not to treat them like brand new hardhats. Follow the replacement guidelines from the date of manufacture, not the date of issue.
If it has been locked away in storage for two years, it should be considered a two-year-old hardhat. And that might mean replacing the suspension before it even gets a single use.
Be Careful with Paint and Stickers
Painting a hardhat or applying stickers to it that have not been approved by the manufacturer seems harmless but the chemical composition of some glues and paints are known to compromise the integrity of the shell.
If a hardhat has been painted or decorated with stickers, consider replacing it since it might no longer provide the level of protection it is meant to provide.