The world may not have come to an end, but your preview copy of Windows 8 soon will. Starting January 15, the Consumer, Release and Developer Preview builds of Windows 8 will restart every few hours and nag you to upgrade to the full version.
In March this year, Microsoft released an update for the preview builds to push their expiration dates back to mid-January -- a little over three weeks away. According to the MSDN article for the update, once the preview versions expire, they'll restart every one or two hours (as ComputerWorld points out, both time limits are mentioned).
Before this happens though -- two weeks before January 15 to be precise -- you'll see this message in Windows 8's Action Center:
This pre-release version of Windows will expire soon. Go to Windows Activation to purchase and install a released version of Windows.
When the fateful day does arrive, you can look forward to constant reminders to update Windows and notifications of impending restarts. As is always the case, I'm sure some enterprising users will find a way to disable the expiration date or push it into the far future.