It is adjustable (from the main settings page after you enable it) but the only options for me are 0 and 5. Zero?

Anyway, they are probably going to introduce 10, according to this article:

http://news.cnet.com/8301-17939_109-10200370-2.html

...but Coleman also told me there's an option to increase the un-send time window to 10 seconds. "We may decide to add longer options," he said.