Several questions come to mind: Are you 24×7 or do you have a “shift beginning and end”
If you have the ability to do the drill just before the day starts, have the agents come 15 minutes early or stay late, sit as they would and then do the drill.
If you are 24×7, you will have to take the slowest part of your day and just cut that time out for the drill. If you HAVE to have numbers available. Pick the slowest volume time, direct a minimum of lines to corless or cell phones and use them to support you during that period.
The entire drill should be done in 15-20 minutes depending on your requirements and number of people. If you have any more questions about this, I would be glad to discuss it in more detail.
Best of luck!
(not sure if they will allow my contact info on here, so I will check back soon if not)