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.
That's right, general rule of thumb for helmet replacement is 3 years or a crash, whichever comes first. In your case sweat and UV damage can degrade a helmet to where it can be unsafe when you need it to work in an accident.
Thanks for the advice. In line with the independent website - http://www.bhsi.org/replace.htm - I was a little sceptical of the blanket 3-year manufacturers rule, thinking it was obviously in their interests to encourage regular replacement. However, based on the latest advice, my faded 6 year old model is definitely ready for the bin so I can now spend even more on cycling accessories, guilt-free!