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As per title really - assuming no major impacts and weekly usage in rain, snow and occasionally sun.
4 years I think? They usually print the date in them..
I've just changed mine after noticing it was dated as 2002!
I think the real answer is no one really knows. Unless someone runs a programme to test old helmets then no one is ever going to know for sure. Anything else is just a best guess and manufactures seem to suggest anything between 3 - 7 years.
Official guidelines vary between brands, for example, Bell say 3 years IIRC. I've stopped using my ~3.5 year old Bell Alchera recently, albeit it's still in the hallway, parked under my new ~£41 hi-vis Giro Savant from Wiggle.