I was somewhat surprised that this information could not be gleaned from the CDR reports, in anything other than one hour increments, so I wrote a quick PHP script to generate the information for any specified time period (seconds, days, months, etc.). Just specify a date and time for the period you're interested in looking at and click the Update button.
Please note that this script was primarily created for my own edification, so it's not well documented, pretty or secure on the open Internet. I would make sure that it's only accessible from your LAN. Just drop this in your /var/www/html/admin folder and access it by going to http://<your PiaF IP>/admin/load.php
The script only takes into account [incoming and outgoing] external calls and ignores calls between extensions.
Checking the "debug" option will display the number of simultaneous calls at every "occurrence" (i.e. every time a call is started or stopped). Use this with care since looking at the debug info for months of CDRs can fill up your browser quickly.
It also provides a distribution of simultaneous calls so you can tell how many times in the given time period you hit a given load.
Hope someone finds this useful.
EDIT: Please see
this post for the most recent version.