Actions configured to execute at logoff may fail to complete successfully if the total processing time exceeds 10 minutes (for example as the result of a large file copy operation or a long running script)
In version 8.1 and above of Environment Manager, the logoff operations are executed under the context of a Local Policy Logoff Script. By default, Microsoft imposes a default timeout of 10 minutes before terminating running scripts and resuming logoff
It is possible to alter the wait time for policy scripts by defining the GPO "Maximum wait time for Group Policy scripts” within "Computer Configuration\Administrative Templates\System\Scripts" section of group or local policy.
The value is in seconds and setting this value to 0 will disable the timeout.
Please note that this policy affects all script processing (such as startup, shutdown and logon scripts)
See http://technet.microsoft.com/en-us/library/cc940027.aspx for further information.