Interest posted question, interesting answers.
My 2 cents... I generally go by the 5 year rule of thumb. I only have one current helmet and it is about 5 years old. I am researching replacement, why? It is showing it's age, the liner is getting worn. I usually replace the face shield every 1 1/2 years to 2 years. It now needs replacing too. I always wear a head a neck liner, which is much easier to wash than the removeal of the helmet liner. Now, I have my old helmet on the shelf. It is now 10 years old. I keep it, just in case I need to replace a part on my current lid and have to wait on a part. The 10 year old helmet I can tell you the liner is wearing out and the foam has given up life, it is hard as hell and uncomfortable to wear, but still better than nothing. And yes I am a rounder, ride about 25K on a bike a year in all kind of weather.
I am also wear a Stich and they say that the suit is good for 10 years or 100K. I have more miles on the suit and it still holds up.
All relative.... just my opinion....not that it makes a difference, but my current helmet is a Shoei X-11 and my old helmet is a AGV modular.