This is actually not an easy task as you need to measure the time the user is logged into the computer. However, it is doable via KiXtart. Create a KiXtart script that does the following:
Check the eventlog for login events fo the monitored users (you need to enable login auditing)
Measure the time the user has been logged in including any previous times for the same day as written into a registry key.
If the user logs out write the time the user has been logged in into a registry key including date.
If the user is logged in longer than the allowed time, then issue a local user logoff command and write the time the user has been logged in into a registry key including date.
Finally, run the script via WKIX32 and SRVANY.EXE from the Windows Resource Kit as a service under an admin account.
Alternatively, get/purchase any of the commercial packages that accomplish the same, see for example http://dmoz.org/Computers/Software/Shareware/Windows/Security/Parental_Control/
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