Initially you will have 60 days as evaluation period, then if you want to extend the Eval. period - Yes you can do, now with the following simple steps.
Slmgr.vbs script.

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.

Micosoft KB here

How to install Windows Server 2008 without activating it

1. How to manually extend the evaluation period
2. How to automate the extension of the evaluation period

Please note : You can extend to a max of 240 days only.