I think 10 seconds would be the sweet spot. I mean, the moment of panic usually comes about 3 seconds after you hit send. leaving 2 seconds for the actual click.
Especially in light of the fact that to be effective, you have to forget the delay is there. Then there's the moment of panic, then there's the moment of remembering that there IS an undo feature, and then the moments to click back to the GMail tab and click.
