MOH anouncement repeat time

Hi All

I have Asterisk 12 installed
I am testing the announcement option in the class definition for MOH

I enabled a test class and set
announcement=queue-thankyou for the test announcement reloaded
Setup a test extension that enables message on hold for the test class

All works, but the announcement happens about every 20 seconds.
Can this interval be changed ?

Thanks in advance

Mark