You can make enable the Undo Send option and choose how long time the Undo button will appear, 5, 10, 20, or 30 seconds.
1. From the main window, choose gear(settings) icon.
2. Then choose Settings.
3. Give a tick at Enable Undo Send (Gmail classic), no need for new version of Gmail.
4. Choose time for Send cancellation period. 5, 10, 20, or 30 seconds.
5. Click Save Changes at the bottom of the page.
Now, when you send an email, it will display Undo button for cancelling, display period according to the time you have set.