There's work currently going on to rebase on to OCaml trunk, show-stopping bugs aside there should be a more up to date version of multicore in the next couple of months - then the focus will be on upstreaming.
Hard to give a reasonable timeframe for release because that's going to depend a lot on how things get upstream.
You can use multicore right now: https://github.com/ocaml-multicore/ocaml-multicore though it's currently at an older version of OCaml, 4.06.1.
There's work currently going on to rebase on to OCaml trunk, show-stopping bugs aside there should be a more up to date version of multicore in the next couple of months - then the focus will be on upstreaming.
Hard to give a reasonable timeframe for release because that's going to depend a lot on how things get upstream.