My experience with a 3 round duration is that I can cast it on a full health mob and still have it go off easily. In theory, a CL 50 Arcane can kill even a 10,000 hp mob in 3 rounds on his own with autoquicken by spamming Avasculate.
Just nuke twice with a K Orb, IGMS or any direct damage spell.
That mob will then explode into a 4,000-8,000 damage AOE with Corpse Explosion.
My personal preference would be to shorten the duration to 1 round (6s) while keeping the damage constant. This means you have to actively watch the healthbar of the mob you're fighting and only fire it on a low HP target.
At 2-3 rounds, it can be cast onto full health mobs and still trigger fairly often.