Well what's your noggin worth? If you can't find any shell deformation, no cracking on the inside it's probably good for some more. I typically replace my daily half shell helmets every 2-3 years unless there have been some hits on them and it gets shortened up. The styrofoam is going to get brittle over time and not absorb energy like a new one. There could be some damage there that your can not see. Many of the good helmet manufacturers offer a crash replacement discount also. I replaced a Leatt Enduro helmet for half the price this year.When to replace a helmet? A few weeks ago I was on a solo ride and took a pretty good spill. I hit hard enough to start seeing stars immediately and it took a few minutes for them to go away. I didn't get knocked out, but did get minor concussion. Looking at the helmet there is no deformation and still looks good, so I have been using it. Just think it might be a good to replace it but I don't see anything wrong with it.
What would you do?