Silent Joel
[12] Conqueror
From a mix of what I've heard and pieced together....
While I don't know when exactly it started, I know that they started pre-planning and planning after they shut Lost Swords down. That shut down on November 30 of 2015. They had announced it was shutting down September 1 of the same year.
Meaning full blown development of SCVI started sometime around early to mid 2016 for sure.
Everything was going well, with them building up and teasing the reveal of SCVI all the way up to the fall of 2016, then the problem(s) arose around November (which were mainly things concerning the style switching/multiple moveset mechanic aspect) which couldn't be resolved by December 2016 by its 20th anniversary, hence the trailer they threw out there with the teasing quote at the end to try and give the fans a wink and nudge to hold them over.
That delay lasted from November to February it seems when I put everything together that I've heard and analyzed as well.
As development did pick back up sometime in or around February no matter what, and they were doing motion capturing from then up until May to play catch up.
Regarding the game itself, it was only planned for PS4 and PC originally from what I heard, but Switch was something they fast tracked development on at some point this year after they saw the Switch's success and they wanted a part of that. Being close with Nintendo probably played a factor in helping make it happen as well.
To get to the point and answer your question, it looks like 2 years so far now, but with a 3-ish (maybe closer to 4) month delay in there at a point.
Do you know why they took so long to start making 6? Most of the sequels came out ~3.5 years later.