I was asked this at the Brisbane VMware usergroup tonight, and in hindsight, I should have blogged it earlier :)

Anyway, by default Windows 2008 gives you ~60 days grace if you install it without a key.  Using slmgr.vbs –rearm you can extend the evaluation up to 240 days.

Check out all the goodness in Microsoft KB 948472 "How to extend the Windows Server 2008 evaluation period", and a snippet is below:

When the initial 60-day evaluation period nears its end, you can run the Slmgr.vbs script to reset the evaluation period. To do this, follow these steps:

  1. Click Start, and then click Command Prompt. 
  2. Type slmgr.vbs -dli, and then press ENTER to check the current status of your evaluation period. 
  3. To reset the evaluation period, type slmgr.vbs –rearm, and then press ENTER. 
  4. Restart the computer.
    This resets the evaluation period to 60 days.