Currently it's 40 seconds, which seems a bit much when compared to the more reliable protection of Zhonyas. Most mages would rather have the invulnerability, and I don't see Banshee's built much except on initiators like Fizz or Diana. The fact that you also have to avoid taking any champion damage for the cooldown's duration makes it almost impossible to use more than once in a protracted engagement, and could potentially extend its cooldown even longer than Zhonyas. I think a duration of 30 seconds, or even 20, would be reasonable. Either that or just remove the clause that disallows champion damage while keeping it at 40 seconds. That way mages might actually make a choice between Banshees and Zhonyas rather than just choosing Zhonyas all the time.