I believe that students would learn as much as if they attended a class with the help of their teacher, however, there many implications that we have to ponder about, namely, the students' learning styles, the encouragement that they get from their instructor, yet, if we take for granted that the instructor is effective as to deliver the online course and that his/her student are highly motivated.
I indeed think that learning would take place, on one hand, due to the format of the course delivery, student would have the advantage of working at their own pace and thus accomplish the intended objectives as well as to have enough time to grasp the course contents.
On the other hand, learners would not have the pressure of achieving certain aims through a more structured deadline because of their learning abilities and styles.