The Windows 7 beta trial program gives you a license that is good for 30 days. But you can extend the license up to 4 times, making you able to having the Windows 7 trial last for 120 days.

To extend you license, you'll need to get a command prompt that is run as an administrator. If you don't know how to do this search for cmd in the Search menu. And when the command prompt is shown in the search results, right-click on it and select Run as administrator.

Once you have the command prompt, enter this command:

slmgr -rearm

Once it completes, you'll need to reboot your machine.

You can use this trick to extend your Windows 7 license and get up to 120 days of the trial. But make sure you use it before your trial expires. It may work after that, but I'm not sure.