Changes to Group Policy Templates don’t immediately apply to systems, because they must first replicate to the domain controller. Also, client systems can take up to 90 minutes to refresh Group Policy Objects. For detailed information, see the TechNet article at http://technet.microsoft.com/en-us/library/cc782584(v=ws.10).aspx.
Administrators can take manual steps to force updates to take effect immediately:
GPUpdate /force