Here are the guidelines from Shoei's website:
Ultimately, the useful service life of a safety helmet is dependent on the intensity and frequency of its use. Helmet replacement is recommended even if only one of the under-mentioned points applies:
1. The helmet was subjected to an impact.
2. The comfort padding or the retention system has become loose due to heavy use or display signs of deterioration.
3. The synthetic foam padding displays signs of heavy use and the helmet feels too loose. Test: with the retention system fastened, the helmet turns to the side when you gently shake your head.
4. There are indentations in the EPS liner and/or white scratches can be seen on surfaces with black paint.
5. Even if none of these is applied, we, SHOEI, recommend replacement in 5 years after it's first purchased at retail.