It won't crumble. I've used one for three or four years now, and it still sparks just fine. It doesn't always spark on the first try, so it might be rubbing off oxidation, but it sparks on the second.

I have heard of cases were the flint did crumble, but in several of those cases the seal on either clorine or iodine tablets had broken. In the cases were it wasn't mentioned, it was mentioned that other items had failed, which makes me wonder if the same thing hadn't happened there. Chlorine and iodine will eat just about anything, so I'm more inclined to base it on very reactive compounds doing funny things to the flint, rather than any oxidation..