it may clutter if it's in words. why not the flower icon(petals changing colours) that's used in dungeon keeper, which is very easy to interpret?
but then, handefood's right. it should be a skill for players to hone themselves to fight certain enemies. trial & error & reload.
plus, spells may be rendered useless for certain enemies so timer's not relevant. in another case where the enemies have resistance to the spell casted which may reduce the effects, either damage or duration.