I found something helpful for you here:
http://www.voip-info.org/wiki/view/Asterisk+config+queues.conf
Periodic announcements are available in queues using the new periodic-announce and periodic-announce-frequency options. This allows a message like "Thank you for holding, your call is important to us." To be played at regular intervals while a caller is in the queue. e.g:
periodic-announce = thank-you-message
periodic-announce-frequency = 60 ; every 60 seconds