A block (spell or mundane both) lasts one exchange, which means it will go away, once you take your next action (which can be another block, of course). I have to admit, I've been pretty dissatisfied with that myself, and I think there was something in the fate core thread about it, a new way to do a spell like that, but that's nothing official yet.
Blocks and armor are two sides of the same spell. A block replaces your dodge roll, while armor reduces the stress you take once you are hit. Enchanted items are enchanted in advance, and you decide on how to employ the spell when you create the item (YS279). That means, that you have to decide if it is a block or armor, you can't change it around once it's decided. You can change your enchanted items at milestones though, so if you are not satisfied with how it turned out, you can switch it there (or at the GMs discretion, of course). If it is a normal spell, you can decide if it is a block or armor when you cast it.