2-3 Month  ROI is very optimistic , you will earn 50$ ETN for 30 days in best case scenario , that is 50% ROI if you started mining at 1st of May .
Once ETN forks at the end of May you will be lucky if you earn 10$ per day .
All of the above without taking into consideration how many  Cryponight miners sold and shipped and not deployed yet .
You will ROI much faster if coin prices increased but then you will make better profits if you trade instead