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):
- Download the exporter from https://gist.githubusercontent.com/yorikvanhavre/2f0e160438b14ac9c31a2fa5117126a2/raw/0183c991449b4946184790e35d48cb294edc6f02/Sketchfab-exporter.FCMacro (right-click the link -> save link as)
- Save it to your FreeCAD macros folder (indicated in FreeCAD, menu Macro -> Macros dialog)
- Run it from that same Macros dialog (select it and press the "Execute" button)
EDIT1 don't download the above file yet, not ready, I'm fixing it...
EDIT2 Okay now it works.