Microsoft's free previews of Windows 8 will expire in January, giving users about three weeks to upgrade to a paid copy or face hourly restarts. From Computerworld:

All three Windows 8 previews -- the Developer Preview of September 2011, the Consumer Preview of February 2012 and the Release Preview -- expire Jan. 15, 2013.

Windows 8 Release Preview, the third and final of the sneak peeks, appeared May 31, five months before the new OS launched in retail on Oct. 26.

Windows 8 previews to go dark in 3 weeks