Strictly speaking, they aren't supposed to have stickers or paint applied to them, as any solvents used in glues or paint systems could have a negative effect on their impact performance. For hats used outdoors, UV will over time greatly reduce impact performance,as may heat exposure. For compliance with AS/NZS 1800, replacement of the harness every two years (or sooner if deterioration is noted during regular inspections) and hat every three years is required.

Sorry, no help whatsoever, but when I saw your post title I started singing along to this https://www.youtube.com/watch?v=LH6MstVxehQ :)

Section 3.4 of AS/NZS1800:
Working Life
Excessive discolouration of the shell colour or weathering of the surface may indicate a loss of strength, as do shells with splitting or cracking of the materials. Such helmets should be discarded.
Attention is drawn to the fact that helmets complying with AS/NZS 1801 are required to contain a safety warning regarding damage due to impact and deterioration.
At the time of issue to the wearer, the helmet should be marked with the issue date.
Field tests have shown that, generally, helmet shells have a life of three years from the time of issue. Components of harnesses may deteriorate more rapidly in service, and harnesses should, therefore, be replaced at intervals not longer than two years.
For helmets that are used infrequently and stored away from sunlight, dirt and temperature extremes, this guideline/recommendation may not be applicable. The user should examine the helmet regularly and discard it if any damage is evident.
Conversely, helmets that are used in extreme conditions or temperature, or poorly stored, may need to be replaced more frequently.

Clearly, it is a recommendation, not a requirement to replace every three years.

Thanks heaps!

For helmets that are used infrequently and stored away from sunlight, dirt and temperature extremes, this guideline/recommendation may not be applicable. The user should examine the helmet regularly and discard it if any damage is evident. — Robb

That's good to know Robb, thanks too

So am I reading this right that there is no definitive number just guidelines?

So am I reading this right that there is no definitive number just guidelines? — Don Ramsay

Yep, because as with everything "it depends"...

As we use two different types of hard hats - one for ground based and one that is required for working at heights we colour code to easily differentiate; we want to know quickly if someone has an incorrect hardhat when working at heights.

The coloured ground based hats used by both field staff and office visitors are deemed to have a 2 year life and we change the colour at the end of each 2 years. While those allocated to office based staff don't see as much work it is much simpler to change them all at the same time so we know that they are current without having to check individual hats regularly

As others have pointed out, standards provide guidance but are not mandatory. However if a standard exists it may well be the premise upon which "reasonably practicable" is assessed. But we are supposedly taking a risk-based approach to managing hazards, which means there is scope to justify (or not) taking a theoretically available control measure such as wearing a hard hat. As Robb has correctly stated there are a number of factors that can shorten or extend the theoretical life of a hard hat. Writing on them with permanent markers, spray-painting them to change the colours, and attaching (as already mentioned) sticky labels with solvent-based adhesive will hasten the deterioration, along with prolonged UV exposure. E.g. the practice of putting a hard hat in the back window of the vehicle is a poor one.
Evolved practice is that a hardhat has a life of two years in an outdoor environment and three years indoor. A useful test is to grip the brims at either side and try and bend the brims inwards towards each other as far as possible. Any significant weakness in the hat will reveal itself by creaking noises and possibly stress cracks appearing. Apply the same test to the peak and the back brim.
I have several hardhats up to 10 years old in my garage. The one in my gear bag is six years old. I apply the bend/pressure test every time I take it out. It is flexible and sound as a bell. I have broken the brim off a hardhat after five years extensive use on construction sites. I took a hardhat from a construction worker who had spray-painted his white hat a bright fluorescent pink and split it down the middle applying the pressure test. It was barely 6 months old. He was gobsmacked, I smiled.
Apply the pressure test, and if in doubt, chuck it out!

I do note lots of road workers wearing hard hats when there is no obvious overhead hazard. In terms of managing hazards, I think they would be much better equipped with cotton some hats with wide brims and neck coverings. A sunhat would be more useful, much cheaper, and last longer than a hardhat that will have to be ditched after a couple of years. A risk assessment completed? I don't think so. More likely there is a "rule" written by someone in an office.

I took a hardhat from a construction worker who had spray-painted his white hat a bright fluorescent pink and split it down the middle applying the pressure test. It was barely 6 months old. He was gobsmacked, I smiled. — Rowly Brown

:up:

Great points, I understand that the hard hat for road workers is historical and seen as best practice, their main threat is traffic striking a worker so possibly o a climbing helmet would possibly be more suitable as it takes the side impacts better than a normal hard hat.

Also intrigued by the helmet that was painted