From the information I have and previous experience I would say 2-3 years.

The 900's 9E is getting are merely an easy fit and nice way to promote what they want. In 2015 9E will be an 81 airframe company. Once all the 200's are gone they will pull the 40 CRJ900's 9E received and send them somewhere cheaper. This leaves 40 airframes and only top earners left, in my opinion this will begin the final chapter for 9E.