Gmail Meter is a Google Apps script developed by Romain Vialard, it can show you stats on volume, daily traffic, traffic pattern, email categories, time before first response, word count and thread lengths.
To set this up on one’s account requires setting up a Google docs spreadsheet and then installing the script on it and requesting a report.
Know Your Gmail Stats using Gmail Meter at: http://gmailblog.blogspot.co.uk/2012/04/know-your-gmail-stats-using-gmail-meter.html
Tutorial: Creating Gmail Inbox Statistics Report at: https://developers.google.com/apps-script/articles/gmail-stats
