It will disapear when you use it. But if you are battling in the PWT, battle subway, or link battles it will only disapear for the battle and come back the next one.
No it does not because the item will dissapear after use so after you finish the battle, the Pokemon will not be holding the focus sash anymore. Source: http://bulbapedia.bulbagarden.net/wiki/Focus_sash#Focus_Sash
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