Originally Posted by
zorak950 fatboy is right, technically whether Google is obliged to update it depends on when it's released: they've committed to OS updates for two years so assuming it releases around the end of the year next year that will be right on the line. They could decide to update it in any case, but there are no guarantees.
It will be at least two years for major OS updates. It could be more, as long as the hardware supports it, like the Nexus 6 and Nexus 9. Every one seems to forget that part.