WIndows 7 Beta / RC Expiration dates

 Windows will notify you that the expiration process is beginning and two weeks later your PC will begin shutting down every two hours.

The 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 PC to replace the OS and reinstall all your programs and data.