Worry no more about typos or replies being sent by mistake. With "undo send" enabled you can bring those replies back and make your edits before sending for good. 

Turn on "Undo Send" under Settings → Personal → Preferences. 

You can set your undo period between 5 - 60 seconds. Then when you reply to a conversation you'll notification appear in the bottom left corner of the screen. 

Click "Undo" to return your message to the reply editor to make your changes.