Hitting anything solid COULD cause them to crack, yes.....In my case, I'm fairly sure it was caused by another arrow slapping the side of it.....

How often you replace them really depends on how often you shoot them, and how much they are "abused". And by that I don't mean you physically abusing them, but hitting things with them. Slapping them together, hitting rocks, trees, stuff like that. If you're shooting good, and suddenly you aren't....That is a good indicator that it's time for new arrows.