18 months of support for Windows Phone 7.8 which will last through the middle of 2014. For now, the update is called Windows Phone 7.x. No matter how many times Windows Phone 7 is updated until it reaches EOL status, there are a few things hardware-wise that it won't be able to match with Windows Phone 8, such as the latter's support for multi-core processors, a microSD slot on some models, the ability to receive OTA updates and NFC support to name a few.
At least with the updates to Windows Phone 7 happening until the middle of 2014, those who purchased their Windows Phone 7 device before the middle of this year on a two-year contract will be able to upgrade to a Windows Phone 8 model before the update well is all dried out for Windows Phone 7.
source: WindowsPhoneRussia, WindowsPhoneLive (translated) via WMPoweruser