I agree that storage is the main consideration - I've had packs of blades go rusty just from having the cellophane wrapper off or the cardboard pack open, kept in a drawer in a very poorly ventilated shower room that suffered from damp anyway. I'm glad to say that the room is no longer a factor as I moved out of the place, but even now I tend not to keep blades in the bathroom at all if I can help it - I keep them in a cigar box in my bedside cabinet now.

I'd imagine that if I'd shaved once with a blade and left it a few days in my bathroom I might get 2 more shaves out of it if I'm lucky (I've got quite sensitive skin so I like to change blades frequently, so your experience may be different). I generally don't put a new blade in unless I'm going to shave, so I'm not sure how long a blade out of its pack and in a razor will last.