Most manufacturers guidelines recommend that your bicycle helmets are replaced every 3 – 5 years.
Giro's advice is the same:
“We make a general recommendation that you replace your helmet every three to five years depending on use and handling,” says senior brand manager Eric Richter. “This is based on observation of the average user and factors like wear over time, weather, handling, the potential for degradation from personal care products like sunscreen or bug spray, and the simple fact that helmet technology does improve over time.”
Clean it with water and soap only, and don’t put any chemical, glue, paint or stickers on it as this might affect the characteristics of the shell.”
Eric Richter adds, “Treating the helmet with care and not exposing it to extreme temperatures will ensure the helmet lasts as long as possible.”
The boot of your car can get very hot in the summer, for example, as can a kitbag in direct sunlight.
You should replace the pads from time to time primarily for safety reasons – they help secure the helmet in the correct position on your head. There’s also comfort and hygiene to think about.
Beware of insect repellents too, in particular, those that contain DEET because, like some other chemicals, they can compromise the construction of the helmet as well as the appearance.