I'm not averse to the idea, but a 10m timer is too much. 3m maybe, 5m tops. A lot of high end spells, if based on the MP cost, would be at least 150 MP back. I think the bigger question is if it would totally negate the spell being cast (on anyone) or if you'd have to be hit by it. The negation would be the bigger perk, really, but then Stun and Silence double for that effect in varying ways.