The Microsoft Life Cycle Support page has revealed Microsoft intends to look after Surface RT buyers for a very long time into the future.
The listing for Surface with Windows RT shows that mainstream support will continue until at least April 11, 2017. A separate post on Microsoft’s support website shows the various SKU builds of Windows 8 will have their mainstream support end on January 9, 2018, with extended support ending on January 10, 2023.
Support in this case likely means the provision of service packs and patches, not necessarily new features, but it still sounds like a much better deal that Windows Phone users can expect, where even for Windows Phone 8 Microsoft is only promising updates for 18 months after release.
Windows RT itself, when not ensconced in a Microsoft Surface, is still to have its sunset date revealed, but in a in an FAQ post, Microsoft says, "Microsoft will make software updates, including security updates, available for Windows RT. Additional information regarding the Windows RT lifecycle policy will be communicated as available."