This year is 2015 or the fourth year of the same design. It's nowhere near long in the tooth yet; even looks stunning with its minimalism against similarly specced laptops like the MSI GS30 or ThinkPad T440s and T450s (Haswell and Broadwell, respectively).
When Skylake is released in H2 2015 (Q3 or Q4) it will be likely that a new design is released. Maybe the high-end mobile chips release in Q1 2016 and that is when new MacBook Pros come out. But think of the ramifications of not having a refresh - late 2016 or Cannonlake, since there's no way an update is coming out with a spec bump mid-Skylake.