That is unfortunately impossible to know As FreeCAD's motto dictates, "it's ready when it's ready". It will certainly take several months more. But:
1) The sketchfab exporter will be available shortly for win and mac in one of the 0.17 pre-release versions available on https://github.com/FreeCAD/FreeCAD/releases (I'll check and notify you guys here when it's updated, we do it by hand)
2) It is already available for ubuntu, using the FreeCAD-daily PPA from https://launchpad.net/~freecad-maintainers/+archive/ubuntu/freecad-daily
3) For older versions, I created a stand-alone version (feel free to copy this and host it on the sketchfab website if you prefer, it's a simple python script with the extension .FCMacro instead of .py):
EDIT1 don't download the above file yet, not ready, I'm fixing it...
EDIT2 Okay now it works.