Back in March 2009, when GMail introduced the Undo Send feature, it failed to impress many. It let users to recall their sent mails, but then they had just 5 seconds to change their mind, which made the feature redundant.
Around a month back, Google improved the Undo Send feature, by allowing users upto 20 seconds to Recall their sent mails. And today, they have made it even better by increasing the maximum time limit to 30 seconds to undo send your mails.
Now you will have the options to set the Undo Send time limit to 5, 10, 20 or 30 seconds. You can find this option under Settings > General tab.
Care must be taken to set a proper value for Undo Send. Say if you set the Undo Send time to 30 seconds, you must note that your mail will not be sent for 30 seconds. This will be undesirable when you want to send something really urgent. Gmail has always took pride in the fact that the mails sent are usually instant, but with this feature you are bound to delay the mail forcibly.
And remember, with third party add-ons like Boomerang, you can schedule the mail to be sent at a particular time in the future. Use the options available wisely.