Windows 7 Beta Bi-hourly Shutdowns begin July 1st, 2009

According to the official Microsoft Windows 7 website March 1, 2010 is the date that Bi-hourly Shutdowns will begin for the Windows 7 RC, so if your running the release candidate, this coming wednesday you'll have nothing to worry about.

http://www.microsoft.com/windows/windows-7/download.aspx

The RC will expire on June 1, 2010. Starting on March 1, 2010, your PC will
begin shutting down every two hours. Windows will notify you two weeks before
the bi-hourly shutdowns start. To avoid interruption, you'll need to install a
non-expired version of Windows before March 1, 2010. You'll also need to install
the programs and data that you want to use.

But if you are running the Windows 7 Beta then get ready for bi-hourly shutdowns to begin on July 1, 2009, and then the eventual expiration of the Windows 7 Beta on August 1st, 2009

http://windowsteamblog.com/blogs/windows7/archive/2009/05/25/clarification-on-the-date-for-bi-hourly-shutdowns-for-the-windows-7-beta.aspx