Office hours can be selected at any time, but how they can be selected to benefit all? The students to ask the instructors and the instructors to have some time to conduct research? Before a class or after a class? In the same days as the class, days with no classes, or all days?
If I had a two day a week class, I would have it before class on Tuesday, and after class on Thursday. And also by arrangement, if someone couldn't come because of their class or work schedule.
At my school, the rule was two hours for each class. There's no recommended amount of time where you are?
I try to set "official office hours" on some schedule that shouldn't clash with classes, but encourage students to ask directly after class if warranted. I tell them that otherwise they can get an appointment, or just drop by. I'll tell them (or leave a note on my door) when I'm unable to spare a few minutes for a quick question.
I think it is important to cover as much time slots as possible. So if classes start for example every two hours, let's say 8-10, 10-12 etc. try to make your office hours for example from 9.30 to 10.30. This way students with a class in one of these slots can still visit you.
edit: Also, if your office hours are directly before or after your class, your students won't have time to prepare for the class after your talk, or think about questions after your class.