The general recommendation in long online classes is to think in blocks or modules of about 15-20 minutes — so, varying the kind of pedagogical approach or activities after about this amount of time. For example, a 15-20 minute lecture, followed by an activity that changes the pace and style for the students. That change of pace can act as a mini-break for the instructor and students.
This block length is slightly shorter than what is generally recommended in face-to-face classes, which often allow for about 25-30 minute blocks. In part, this is because the online environment, especially when it is new, may place additional cognitive loads on both faculty and students – so allowing short periods of focus and varying activity can help balance that heavier “lift.”
In terms of actual breaks: having at least 1 complete break from activity in the 3-hour period would be highly recommended; some instructors might like to include two shorter breaks — I think it depends on the material that’s being covered in that class and how the instructor structures the blocks overall.
Break times you typically see in a 3hr class are usually between 10-15 minutes, but no more than 30 minutes total. In my experience, it’s a balance between wearing students and faculty out and losing momentum with breaks. You also need to factor in any transition in technological use — e.g. moving into breakout rooms.
Here’s a possible structure for you to consider (and you can find the original version of this from RIT here). You should feel free to play with the length and type of segments to allow for the material you are covering and the kinds of activities you have planned.