ClonePrep post-synchronization or power-off scripts have a timeout limit of 20 seconds. You can increase this limit by changing the ExecScriptTimeout Windows registry value on the golden image virtual machine.
Instead of increasing the timeout limit you can also use your customization script to launch another script or process that performs the long-running task.
Note: Some ClonePrep customization scripts can finish running within the 20-second limit. Test your scripts before you increase the limit.