How low long does the effect of spells with AoE in battle remain?

For example Incinerating Cloud does stay for at least 6 turns. (At least the animation does, don't know about the effect.)

The spell description in the spell book doesnt say much about the duration...
While there is a information / point called "Duration" for every spell, it does - for me- not make much sense.
Incinerating Cloud has a duration of 1 according to the discription. So I would guess it means 1 turn only..., BUT as stated above the effect stays at least 6 turns...