Just in case a constantly increasing unread messages counter isn't providing enough details about the flow of messages to your inbox, the Official Gmail Blog has pointed out the Gmail Meter tool for detailed monthly activity breakdowns. 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; providing an even deeper dive than Google's own Account Activity dashboard. Setting this up on one's account requires setting up a Google docs spreadsheet and then installing the script on it and requesting a report, there's a YouTube video embedded after the break and a tutorial linked below to help you along. On the plus side, now when someone asks "why haven't you responded to my email yet?", you can show them they're still well within the average time before first response window.