My money is on another four years of JB. The man is in better shape than me, and I'm 28. It's anyone's guess, but maybe he does give the reigns to Hop in 18-19. It could be a factor in why we can't get early commits for next year. With all the playing time in the land. Just an uninformed speculation.
Regardless, this year is his best shot to cement his legacy. Most of us know that he's one of the greatest. He needs another Championship to get the naysayers off his back. He's a competitor.
When he thinks he can't win, then he will step down. I don't think the announcement is set in stone. Though, I feel that he respects Hop so much that it may be so. Who knows. Personally, I'd love to see him another 5 years, or more. Or whatever the man wants to do. He has a family, and a life outside of basketball. Whenever he does hang it up, will be a sad day. But, it's gotta happen some time, and he will always be a legend.