You should replace your helmet after any significant crash, whether the helmet has visible damage or not. Helmets are a safety device designed for single impacts and should not be used once damaged.
Cracks or dents can happen, although rare, from normal wear and tear. If you notice your helmet has one, you should strongly consider finding yourself a new brain bucket.
Your helmet manufacturer will have a specific recommendation on how long your helmet should last, but we tend to see recommendations anywhere from 1 - 5 years. In general, over 3 years is a good time to start looking for a new helmet. Storage conditions, weather, degradation from sweat, sunscreen, or bug spray all are determining factors in how long a helmet will maintain protective capability.
