http://www.slate.com/articles/video/the_short_cut/2015/07/imessage_timestamps_swipe_left_to_reveal_when_a_message_was_sent.html

In the current Messages app (iOS 8), iMessage and SMS conversations by default are divided into large time chunks. You may have noticed the automatic chronicling by day or week, depending on how much you message another person. However, what if you wanted to see the exact time a specific message was sent? In the video above, quickly discover the timestamp that Apple has hidden by default in iOS 7 and 8.