Lifted from Microsoft:
"Evaluating any version of Windows Server 2008 R2 software does not require entering a product key, however will require activation within 10 days. Failing to activate the evaluation will cause the licensing service to shut the machine down every hour (The 10 day activation period can be reset five (5) times by using the rearm command. See below for further information on activation rearm). Once activated, the evaluation will run for 180 days. After this time, you will need to uninstall the evaluation software and reinstall a fully-licensed version of Windows Server 2008 R2"
Looks to me like you have to 'activate' the software in order to begin the trial period.
(The following paragraph lifted from blogs.technet.com here)
"Please note, however, that while product activation is required, the product key is actually built into the software so all you need to do is click 'Activate Now'. "
In effect you could get a 230 day trial by 're-arming' 5 times before clicking the 'activate' button