How often do I have to activate products of MS Campus EES?

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You can check the remaining time with the command slmgr.vbs /dlv. In the section "Time remaining", you can see the how long Windows stays activated. Usually after a "fresh" activation, this time should be ~180 days.

The re-activation is also possible via VPN to the MWN network.

One note: an activation is only allowed for computers, which are in ownership of the TU München and with a existing basic license.