Windows 8 Developer and Server preview are currently set to expire on March 11 and April 8th 2012. But now Microsoft released an update (KB2671501) to both previews to extend the expiration date to next year, 2013. The update can be installed by going to Control Panel and clicking Check for Updates. After installing the update, the computer has to reboot in order for the new expiration date to be extended.
Regular consumer can expect to have a preview soon as well, as developers currently do. The preview for consumers (Windows Consumer Preview) will be available for download on February 29.