The RC will expire on June 1, 2010, giving you more than a year to test it but one thing previous reports didn't tell is that starting on March 1, 2010, the Windows 7 RC will perform bi-hourly shutdowns. This is Microsoft's way of telling you that it's time to pay for the retail version.
To download the Windows 7 RC ISO you'll need to login with your Windows Live ID and fill out a form. The 64-bit version weighs in 3.05GB.