Gmail Meter is a script designed for Google Docs so that you can get to the bottom of how you communicate. It will tell you how many emails you've received and sent, and what your email traffic looks like each day, with word count, and how long it takes you to respond to each message on average. The instructions to set up:

Go into your Google Docs account and make a new spreadsheet. Call it "Gmail Meter."
Go to the Tools menu and choose Script Gallery.
Type "Gmail Meter" in the search box and click the Search button.
You should only have on result: "Gmail Meter" (surprise!). Click the Install button next to it and authorize the script when prompted. Once you authorize, you'll also need to grant access to the script, so go ahead and do that.
You'll now see a "Gmail Meter" menu option. Click it, and choose "Get Report."
When prompted for what type of report, you should probably choose "Monthly report" to start rather than a custom one, but it's up to you.
The script will start generating the report. It may take awhile, but you'll receive an email when it's ready. Just set it aside and come back when you're notified.

Check out a video of it below: [Official Gmail Blog]