Windows 7 Build 7000 (Beta) expires on August 1, 2009, and bi-hourly shutdowns will begin July 1, 2009. The RC will expire June 1, 2010, and the bi-hourly shutdowns will begin on March 1, 2010. In both cases, you’ll need to rebuild your test PCs with another valid version of the OS, and reinstall your programs and data.
With that to say, if you don't already have the RC version installed and do choose to install it (which I would highly recommend), you will have free use of it for a little over a year.
For more information head over to: http://tinyurl.com/832nco for videos, discussions and further updates about Windows 7 RC.