Commentary from the Desiring God web site for Easter:
Crucifixion in the ancient world was intended to take as long as possible. No vital organs were damaged, so it took two or three days to die, often from shock or asphyxiation, as muscles used for breathing grew weak.
Luke 23:39–43 is a conversation between Jesus and the criminals crucified alongside him, and it is in the Bible because crucifixion was slow. There was time to talk. This conversation is surely one of the most extraordinary in the Bible. It shows us the similarities of these three dying men, and yet, at the same time, how very different Jesus is.
I encourage you to read the rest of the article:
http://www.desiringgod.org/blog/posts/jesus-between-the-criminals
Sapphire Sky 