You didn't actually defeat your opponent. You used a card that granted you a win and massively shifted the balance of play.
That being said:
If this were to be implemented, I think 7 turns would be a good starting point.
I also think this would be like a doomsday device card with a time bomb like effect. The benefit of that is that it can be defused with PC.
I guess, but then the card would have to be cheap or have it's turn counter reduced because it would be too UP. It wouldn't make a whole 25
card getting defeated by 1
or maybe you LOSING because it got stolen from you.
You'd have to do a cheap earth splash just to protect it, or use cloak.