The Gmail Undo Send option which was introduced early last year is a very handy feature that allows you to prevent a mail from going through if you hit the send button accidentally. When this option is enabled under Gmail settings, Google will wait for a few seconds before sending an email. If you act quick enough you can cancel the mail by clicking on Undo Send, and avoid the embarrassment of sending the wrong email.
Initially the delay was set to either 5 or 10 seconds. Recently, Google made this better by offering up to 30 seconds of wait time. Users now have the options to set the Undo Send time limit to 5, 10, 20 or 30 seconds.
So if you are of the slow kind, you can set the time to 20 or 30 seconds. Be aware, that every email you send will get delayed by this time. This will be undesirable if the mail is urgent. In my opinion, 10 second is enough reaction time.
[via Google Operating System]