The Surface 2 and Surface Pro 2 have only been available for a few days, but Microsoft has already revealed details of the support lifecycle for the devices. Buy a new Surface 2 now and you can expect to benefit from mainstream support until 10 April 2018 according to the Microsoft Product Lifecycle page.
This lifecycle applies to the Surface 2, Surface Pro 2 and the Chinese version of the Surface Pro 2. Interestingly, the lifecycle start date is listed as being 20 January 2014 so it would appear that Microsoft is not actually supporting these devices until next year -- this is unlikely to actually be the case!
The Product Lifecycle page also shows that the support date for the Surface RT (or the plain Surface as it is now known) is 11 April 2017, while owners of the original Surface Pro benefit from support until 10 July the same year.
Interestingly, support for the Chinese version of the first generation Surface extends to 10 July 2018 -- a full year longer than the Surface Pro.
But whichever model you have in your hands, you have at least four years of support left.