usually say that you should send in your helmet for "evaluation: after one blow to the head. A good DH helmet can cost upwards of 350 dollars, with a good half-shell starting around 125, so there is a financial incentive for riders to downplay the severity of their crashes in order to stave off a new helmet purchase, or to avoid the long turn-around to send the helmet to the maker for a look-see.
Many (if not most) riders keep a mental inventory of their crashes and arbitrarily choose a point where they believe it should be replaced. If hanging around the chair lift line at Whistler provides any indication of average helmet usage, a lot of riders don't buy a new helmet until the stink becomes too overwhelming. Take a moment and consider where you stand among those extremes, and weigh the lifespan of your helmets against the number of times the two of you have hit the dirt.
Inspecting a helmet for internal damage is not easy, because it's the foam liner that does the lion's share of dissapating the impact, and the part that compresses permanently is under the hard shell, not near the head where the deformation can easily be seen. Sometimes, the damage can be located as a "soft spot" in the flexible shell of a half-coverage helmets, but there is no chance of determining damage to the liner of a hard-shell, or full-face helmet.
The point is, it's difficult, if not impossible for a wearer to assess damage, so unless you manage to smack your head in a completely fresh section of your helmet EVERY time you go down, you should replace it after one whack. So, today's poll is: