How to trigger Active Directory update?

I am attempting to set up FreeNAS with Active Directory. My one concern is that when I update a user in the domain controller, it appears to take a few hours for it to show up in FreeNAS. Pressing the Rebuild LDAP / AD Cache button does not appear to make any difference. How would I speed this process up? If I add a user, I need FreeNAS to add them fairly quickly.

Thanks for you time,

Ian