A company spokesperson told me that there is no deadline for installation on the discount licenses. In other words, if you buy a $40 license today, you'll be able to sock it in your pocket for later and upgrade your PC to Windows 8 Pro a year (or more!) from now. Multi-PC households take note: Microsoft allows you to buy up to five upgrade licenses at the $40 price point.
You might want to get around to upgrading to Windows 8 sooner rather than later if you plan on using Windows Media Center, though. Unlike Windows 7, Windows 8 doesn't include the Windows Media Center software (or DVD playback capabilities) by default. In fact, the Windows 8 Media Center Pack will actually set you back $9.99--but only if you don't act quickly. If you're running Windows 8 Pro and ask for a product key for the Windows 8 Media Center Pack by the end of the month, Microsoft will send you one absolutely free. You'll have to install WMC immediately, however. The same Microsoft spokesperson told me that "The [WMC] product key must be activated no later than January 31, 2013 or it will not be valid."
What if you aren't upgrading?
This doesn't really have anything directly to do with the upgrade offer, but if you're planning to custom-build a PC anytime soon and have your heart set on Windows 8 Pro, it might just be prudent to buy a stand-alone, non-upgrade license now. Currently, Windows 8 Pro System Builder--the version needed by DIYers--retails for $120 to $130 online.
I asked Microsoft if the cost of Windows 8 Pro System Builder would increase on February 1, as the price of a Windows 8 Pro Upgrade license is set to jump to $200 on that day. I was told that "Microsoft doesn't publish estimated retail pricing for the System Builder product; it is up to retailers to set their own ERPs depending on the amount of margin they would like to make on the product."