It'll almost surely take less time, part of the reason 0.17 took so long was because the Part Design Workbench had compatibility-breaking changes with a whole new workflow. It's probably overly optimistic but some of us are pushing to have this development cycle last roughly until the end of the year before we begin the release process again. At the earliest, that'll probably mean another release this time next year.
Estimates are hard enough in the professional world where one pays X developers to do Y. In a volunteer-ran open source its nearly meaningless...
Start to use it today, use extensions like Assembly2/3 to compensate for missing features and do your best to help out if that is not enough. Or have patience :)
Oh yes, I'm well aware of this. I wouldn't still be using GIMP after 10 years if I wasn't a patient person. :)
Though I fiddled with an example file for ~5 minutes and it crashed twice in that timespan. Exactly like the last time I tried FreeCAD a few years ago. But I would really like to move on from F360 so I will try to give it more of a chance this time, and hopefully I'll find the time to contribute something.
There's a roadmap page but it lists 0.18 as "Scheduled For Release 2017-11-03".