Are you asking about hardware support (whether repair and spare parts will be available) or software support of the hardware (whether new OS X versions will run on it)?
Apple is pretty clear about the sunset period on the hardware side:
"Owners of iPad, iPhone, iPod or Mac products may obtain service and parts from Apple or Apple service providers for 5 years after the product is no longer manufactured (or longer where required by law). Apple has discontinued support for certain technologically obsolete and vintage products."
"Vintage products are those that have not been manufactured for more than five and less than seven years ago. Apple has discontinued hardware service for vintage products with the following exceptions:" [Macs bought in Turkey or California]
"Obsolete products are those that were discontinued more than seven years ago. Apple has discontinued all hardware service for obsolete products with no exceptions. Service providers cannot order parts for obsolete products."
(excerpts from Vintage and obsolete products - Apple Support)
Less so on the software side. Mountain Lion, Mavericks, and Yosemite (and El Capitan, I think) all support Mac Pros back to and including Early 2008 models. The reason commonly cited for that being the threshold is that Mac Pros before then had only 32-bit EFI boot code and OS X requires 64-bit EFI. Nobody outside Apple knows what hardware requirement might arise in future releases of OS X.