Well. How long did they need to bring the A319? That would be nearly the same, if not even more regarding the timeframe.
And even if Airbus controls the CS series, there is still a hell of a difference between the CS series and ANY Airbus. I think, we won´t see the CS series done by FSL and I also think, FSL will decide not to develop the A318.
At least they wouldn´t get my money for this bird. And many share my opinion.