You'll need to create an Adobe ID at download time or within 7 days. I couldn't find any time limit so if you want to try Photoshop for free here's your chance.
A caveat for XP users: * 3D features and some GPU-enabled features are not supported on Windows XP.
In the FAQ at Adobe: When will the Photoshop CS6 Beta expire?
Answer: The Photoshop CS6 beta will expire soon after the launch of Photoshop CS6 in spring 2012.