When I first heard about Windows XP downgrade rights has been extended to 2020, I was thinking how Microsoft can commit to the long duration. It seems that there are some confusion over the exact date, as mentioned by Microsoft. And Microsoft is also not very direct with their explanation to this matter.
According to Microsoft Windows Blog, it states that the downgrade rights will stop 2 years after the next Windows OS (presumably Windows 8) is shipped.
Customers who purchase Windows 7 PCs with end user downgrade rights as provided in the software license terms (EULA) will be able to downgrade to Windows XP Professional on those PCs for the life of the PC. However, customers will not be able to buy a Windows 7 Professional or Ultimate PC with end user downgrade rights after Windows 7 reaches the end of sales date in the OEM channel – which according to the current Windows Lifecycle policy is 2 years after the next version of Windows ships.
That means there is NO exact date to the extension. However, support to XP will stop after April 2014. Anyway, if you are already happy with Windows 7, these information will not matter anymore.