|It'll take how long?|
The ROI Smith is talking about in his post is the length of time and words it takes to write a short story, short novel and long novel and how much money you'll make in return. He includes calculations for how long time-wise it will take for you to make enough money to cover your time investment. Those time lengths may surprise you. Although Smith gives the time in months, it's actually years to cover your writing time investment.
If you want to determine where your writing time will be best spent, tootle over to Think Like a Publisher 2013: Chapter 5: Return on Investment