When will the Windows 7 Release Candidate expire?

The RC will expire on June 1, 2010 as stated on the Microsoft web site.

"We can now install Windows 7 Release Candidate

Windows 7 was actually officially released July 21. If you have an MSDN subscription, you can download it now otherwise official shipping media should be available starting October 22. If you haven't already installed the RC, you are probably better off waiting until you get a copy of the release and install that.

information about when it will expire, or can I run it forever?

The RC will expire June 1, 2010, and the bi-hourly shutdowns will begin on March 1, 2010. In either case, you'll wantto rebuild your test PC to replace the OS and reinstall all your programs and data."