If you are using the demo version Hyper-V image (eventual with VirtualBox) you can re-activate the license up to 3 times using the rearm script
In order to "rearm" Windows on the image, open a command prompt (Start / Run / Cmd) and type the following and enter:
It will eventually tell you that re-arm was successful and to restart your computer (the image). After you've restarted the image, you will need to activate Windows again to continue using the image.
Go to Start and in the search field type in 'Activate Windows' and follow the instructions. (you will need to be connected to the internet from your image)
That will give you a licence for another 180 days.
On the newest image you can just click the "Rearm windows" script in C:\Program Files\Microsoft\rearm