Hi Trent,
This can be changed via Class of Service Greetings Settings.Set up a COS that allows the length of Greeting that these users need or just adjust the existing COS that they are using;
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
Select the recording length allowed to subscribers assigned to this COS for recording greetings.
The range is 1 to 999 seconds; the default is 90 seconds.
If enabled, a warning tone will sound before the maximum greeting length is reached
From this good doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_administration_guide_chapter09186a00801188cb.html#1057646
Hope this helps!
Rob