Rear glass can be laminated and joined with a piece of glass.  Sometimes this process will have vinyl film in between.  This will help prevent the rear window from shattering even in a crash.  Rear windows are made of tempered glass — glass that has been heated to extremely high temperatures then rapidly cooled.

How long does it take to replace a rear windshield? – (rear window or back glass)? From removal of the damaged glass to the installation of the glass.  The entire process should only take a hour.  The vehicle will need to sit an additional hour depending on temperature.  Each season will vary on time.