An interesting question is whether a retirement announcement at the end of next year's term could be use to get out the vote for a replacement of similar political stripe.
I think there's a general consensus on the Court that there should be no retirements in a presidential election year (obviously with the exception of extraordinary circumstances). If Hillary (or another Democrat) wins next year and Democrats take back the Senate, I wouldn't rule out retirements from either Ginsburg and/or Breyer in 2017 or 2018. While it's possible Kennedy and/or Scalia could retire during the next term, I think 2020 will probably determine their replacements.