According to a member of the dev team: 'We are aiming for sometime in the second half of the year. There's still a few details to work out beyond just feature parity with 15. '

https://www.reddit.com/r/reactjs/comments/6g3e98/react_fiber...

Huh, seems like only yesterday there was a good number of red tests: http://isfiberreadyyet.com/

it's awesome the whole refactoring process they have done in such a short period of time. Short, relatively, but short.