Should remote teaching sessions be shorter than classroom sessions?

If you search using Google or another search engine for something like "research on maximum effective lecture time" you will get some ideas. In general, long lectures are less effective than short ones because of the student's attention span. Video lectures should be very short, say 10-15 minutes for effectiveness. And with required activities at the end of each short session. So, a long "lecture" can be presented as a sequence of short but complete segments. Real time is a bit different, but keep all of that in mind in course design.

But it really depends on what you and the students are doing in the given time period, rather than the length itself in any absolute sense. A talking head writing on a board or (worse) showing a bunch of slides gets old and boring very fast. If the students have a chance to break it up with questions or some problem solving activities it is better and the class-period can be longer and still effective. Class discussion sessions is another way to break the monotony, but harder online and nearly impossible with video unless there is an effective communication channel shared by every student.

I used to teach a couple of courses that lasted all day and only met once a month. But the face time was usually broken up into small segments and we used very effective, but simple, communication tools so that students could "engage" with the course and the instructors (two of us) at any time through the period between face-time sessions.

The key is to keep the students active and engaged. Once they lapse into passivity you are lost. Some students have the skills and background to keep themselves engaged in sub-optimal environments, but not all do. Perhaps not even the majority.

No research here, just a small amount of experience to chip in. :) And no answers, just observations...

First, they're coping with many changes that roll in on the news daily or weekly, many of them more of an upheaval than taking the same number of instructional hours and cutting it into smaller blocks. I don't think you should decide based on unfamiliarity in COVID times.

In my experience, your intuition is right: 90 minutes is a long time to be continuously focused on Zoom, but 45 minutes is very doable. Worse, many people (try to) multitask during remote learning, and 90 minutes is a long time to be dividing attention.

On the other hand, I think a longer break is a good idea. Students seem to find it hard to really break on Zoom. They turn off their mic and video or run to the washroom, but it's hard to do the necessary dissociation: standing up, moving their legs, bustling and talking and sharing impressions with peers, getting a breath of fresh air. If they stare at a screen during break, they won't come back with a fully refreshed ability to sustain attention for another block. For this reason I suspect that 30 minutes of break with explicit reminders to get up and away from the computer is more useful than 15 minutes. But 15 is better than 10 and 10 is better than 5.

As for coming back late and wasting time: not your problem. It's no different from deciding to get a snack and wander luxuriously when learning in person. If they're not back on time, the lesson doesn't stop for them.

How about the problem of momentum? Four blocks with three breaks between them is pretty significant as far as stopping and starting. You would need to be very much on point and purposeful to avoid transition/inertia eating into the 45 minutes from both sides.

One idea is to designate each 45-minute block for a different purpose. For example, if all four are direct instruction, the repeated pattern will accentuate the dreaded feeling of sameness. "Is this the third block or the fourth? Did he teach us that earlier today or last week?" Whereas if you teach the first 45-minute block, give them exercises the second one, teach the third, and give them exercises the fourth, that's quite a nice rhythm.

The same strategy also helps mitigate a 90-minute block. Being present and working while the instructor supervises / is present for questions is not taxing; what's taxing is being attentive while the instructor talks and shows slides (on a screen you may not be viewing or have snapped to half of the screen).