When I cast a spell that has a duration, such as Fury, I want that duration to vary according to the difference between the spell magnitude and the enemy level.

For example, Fury has a magnitude = 6, and duration = 30 seconds.
If I cast this spell on a level 6 enemy, this enemy will frenzy for 30 seconds. But if I cast it on a level 10 target, that target would frenzy for only, say, 10 seconds.

Any idea how to accomplish this through a script?