It depends on a person's background. There are people like Jeff Heaton who have years of professional experience in Data Science taking an introductory data science course, and there are invariably people taking machine learning with no programming experience. But in the sweet spot for any class there will be people who are really stretched. People who will spend twenty or thirty hours on a programming assignment that some people complete in one.

I enrolled in the last iteration of Van Hentenreck's Discreet Optimization. A great course and his enthusiasm is infectious. I learned a lot. Saw where I needed to go. But there was no chance I was going to pass. I just don't have the chops...yet, hopefully.

One of the things that makes Discreet Optimization a great MOOC course is that it can be approached at different levels. A student can attack the problems using dedicated optimization libraries. If that's not enough of a challenge, they can write their own algorithms. And if that's not enough, they can prove optimality for each of their solutions.

And like every Coursera computing course there are people who can do all of it. And most who cannot.