I was wondering how long the white magic spells that affect evasion, attack or defence (such as INVS, FOG, RUSE, etc) last in the first Final Fantasy? Do they last for an entire battle? Of just a limited amount of turns?
In most cases status effects will last until either the end of battle or when the character dies.
Elemental protections (AFIR, AICE, etc.) can also be removed from allies with the XFER spell. XFER doesn't work against enemies.
With the black magic spells SLOW and FAST, which half and double the number of hits of a target respectively, they can basically cancel each other if they both hit a target.