The editor for One Piece (Weekly Shounen Jump) revealed some interesting details on a stream called "School of Jump". One Piece will enter Wano Country Arc next year (2017) and that the Whole Cake Island arc is now half-way complete. These details were revealed by the editor (not by Eiichiro Oda), and there is no exact dates mentioned. But we can expect things to unravel at an exciting pace (the editor mentioned that the remaining part of the arc will be more fast-paced). Taken from another forum.
It's almost a bit concerning that Cake Island is halfway through. But I suppose if it's close to the battle or climax, that's alright. Hopefully Oda wraps it up nicely and it's not rushed or anything. Wano should start between March and May?