I graduated from the MDiv program at HDS this past May and would say that you should expect to be on campus for 15-25 hours a week for classes and the like. Lectures tend to meet for more course hours than seminars (lectures typically are twice a week for 1.5 or 2 hours and then have hour long discussion sections once a week, whereas seminars are typically 2-3 hours once a week) and language classes often meet more often than that (depending on the level of the language course you may have class three times a week for an hour or two). During shopping week you get to go around to whatever classes you're interested in, both in the yard, at other graduate schools, and at HDS, so you can look at the syllabi, experience the teaching styles of your professors, and sample what discussion might look like. So you can have a great deal of say in how much time you're in class/on campus each week, reducing it as low as 2 or 3 days a week if you're savvy. That said, there is a lot of work outside of class, so to be successful you should plan on committing at least 50 or 60 hours a week to your duties as a student. I've seen people slim it down to just 40, but that often comes with a ding to their GPA and the overall quality of their work and engagement with the materials.
Finally, HDS and Harvard have a TON going on in terms of student activities, clubs, conferences, etc. and I highly encourage you to find at least one or two things and get involved. In my experience, the students at HDS will be your greatest teachers, so taking the time to push beyond the culture of business and forge real casual and collaborative relationships with other students will be well worth the effort.
Looking forward to seeing you in the fall too, Pertyion!!