You are probably going to get a lot of folks saying if it ain't broke don't replace it.
In my experience I've gone with what the helmet manufacturer's suggest, which I believe is every 3 years. In fact I just got a new Ionos to replace the one I got 3 years ago, after cracking my old Atmos in 3 places during a crash. The foam lining will break down over time and a LBS once told me that the more advanced helmets, which do cost more, will generally last a bit longer than lesser priced ones. That being said, my new Ionos was purchased because of the 3 years of riding I had in it, but also becaue I was tired of the plain white color of the previous one.
I'd recommend checking yours for cracks in the foam lining and ask around at some LBS.