Windows 7 Beta Clients will expire on August 1st, 2009. In the latest subscriber mail Microsoft stated that they will begin bi-hourly beta shutdowns in June.
According to Microsoft spokesman Brandon LeBlanc this is incorrect. The beta shutdown will not start until July.
Therefore, I highly recommend everyone to upgrade as soon as possible. I, myself still have to do it on 1 PC. Here are some instructions: Beta to RC Build Update
For clarification, once they start the beta-shutdowns your PC will begin shutting down every two hours.