Microsoft Support Lifecycle website confirms that the company will end the mainstream support for Surface with Windows 8 Pro in July 2017 (July 10th, 2017 to be more exact). This is just over 4 years and about 5 weeks shorter than the mainstream support for Surface RT that the company announced back in October 2012.
During the support cycle, Surface Pro gets free hardware repairs, but limited to the warranty period, and support over the phone throughout the four plus years lifecycle. The company also makes available parts and replacement units during the warranty period as well as software and firmware updates.
Coincidentally both devices do not have an extended support expiration date as you’ll find in other Microsoft products (software), which is typically five more years.
However, because Surface Pro comes with Windows 8, the support for the operating system will continue through mid 2018, follow by the extended support that is set to end on mid 2023. As for Windows RT and Office Home and Student 2013 RT, Microsoft has yet to reveal the mainstream and extended support dates or if they will be applicable at all.
Source Microsoft Support