[K]

The general rule is that damage is an instant effect that takes place when the spell it cast, and any extra effect from a spell (like Decay, or burning from fire, etc) last either for the rest of the battle or until some kind of save. For example, Banefire's damage is instant, but if the the Decay effect hits then it lasts the rest of the battle.

Some spells let a unit get rid of the effect. Being set on fire, chilled by certain frost effects, and being tangled in vines are all common effects like this.

Buff spells generally last the rest of the battle. The exception is "Battlefield" or BE spells that affect the battlefield and not the troops (look for an icon on the top right ogf the battle screen). These spells last until the end of the battle OR until the caster is killed. An example is Quagmire.

There are buff spells that only affect all your troops and not the enemy troops, or just enemy troops, and these last even if the mage is killed. Will of the Fates is an example.