mboxstats[1] is pretty good for this if you're fetching the monthly archives from the web interface and desire a quick overview. It supports mbox and maildir, and can produce XML output for easier post-processing.

The mboxstats author has a variety of interesting tools on his site, if you're in to strange stats and log processing like me ;)

I've submitted it here[2] in the hope others pile in with other cool options.

1. http://www.vanheusden.com/mboxstats/ 2. https://news.ycombinator.com/item?id=13418463