One of the useful features of email is being able to see exactly when an email was sent. However, unlike Mail, the default view in Messages doesn't show the time when messages in a thread were sent. Messages are time-stamped just as emails are—you simply need to know how to view the time stamp.
The screen shot below shows the default view with no time stamp for the replies in the thread.
In order to see when all the messages in a thread were sent, simply tap and drag a bubble to the left, and the time stamps will be revealed.