No idea. Not even Arc himself is sure.
One or two years, and that’s lowballing it.
Arc doesn’t intend to work on the game for a decade.
You can work on the same thing for so long before you get bored of it.
That’s why the game after CK won’t be the actual CK sequel, but something different.
But yeah, projects like these, it’s hard to do estimations of time.
And it’s really easy for things to snowball out of control.