In this paper, the authors report results of a large-scale study of more than 2 million users exchanging 16 billion emails over several months. They quantitatively characterize the replying behavior in conversations within pairs of users. In particular, the time it takes the user to reply to a received message and the length of the reply sent. In addition, they study how increasing load affects emailing behavior. Finally, they predict the time to reply, length of reply, and whether the reply ends a conversation. This study directly impacts IG strategy on email.