Hi Chris,
In Unity the Greeting Length for a Call Handler is determined by the COS of the person recording the Greeting. I believe that this config also carries across to Unity Connection. Have a look;
Class of Service Greetings Settings
The class of service greetings setting specifies the maximum length of greetings.
Subscribers > Class of Service > Greetings Page
Maximum Greeting Length (in seconds)
Hope this helps!
Rob