Expectations for a research student at a Japanese university
I was a research student at the University of Tokyo for one year.

As the answer to the other question states, a research student has literally no responsibilities whatsoever. The apparent point is to allow foreign students time to study for the entrance examination for the formal graduate program. They might give you as little as four months (roughly) or as much as a year before the exam is held.

Your advisor may or may not recommend that you sit in on classes and learn how the university works. If you are in the sciences, there's a good chance that you will be assigned to a research team in preparation for entering the college. If you are in the humanities, it's likely that your only responsibility will be to try to pass the examination. If you are already familiar with your field, you will have an extreme amount of free time.

"Research student" is actually kind of a misnomer. During my pre-acceptance year I did independent research on my own initiative (mostly because I was bored) and had two papers accepted for publication, but my advisor did not ask what I was doing, and my office did not know I had published papers until after I was accepted as a masters student.