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TL;DR, 3-5 years mostly based on how much/how big I crash in the helmet, and I store it indoors in a dark cabinet so it doesn't degrade. I think the official line is that helmets last 3-5 years, but it's meant to be 3-5 years of active use. If you are storing your helmet in a dark, cool, and dry place it can probably last for longer as the material isn't degrading as much. That being said, I usually replace my helmets at the 3-5 year mark of ownership anyways since I generally have crashed in them at that point...

I just get a new helmet every 3 years or after a crash. Helmets aren’t that expensive in comparison to bikes and there’s just no point in taking a risk when it comes to your brain. I’ve had a heavy concussion with partial amnesia once and it’s not something I ever want to go through again so it’s worth the 50-60 bucks a year to know you are on top of your safety gear.

There are a few highly rated (https://www.helmet.beam.vt.edu/bicycle-helmet-ratings.html) helmets that are $60-100. That's what I go for, makes it easier to replace after even a mild crash than a $300 helmet. And for open face helmets I've never really noticed much difference in terms of fit and function between expensive and reasonably priced ones. Think of helmets as a wear item, 3-4 years or after a mild crash.