The extension letter will be sent to you immediately after filing an I-751. Getting an ADIT (after extension letter expires), entails calling USCIS, convincing them to grant you an appointment, them going to a USCIS office to ask for the stamp. Recently, numerous people have reported being refused an appointment. It's a mess. The new 48 month extension letters will help. But extension letters are only good when employers, airlines, etc understand that they legally extend the Green Card expiration date.