There was this study timetable I saw, that had four subjects fixed on one day. I knew straight-up, there was no way that person studied all those subjects, and fully understood everything they had passed through. And it had nothing to do with the person’s seriousness or commitment.
Let’s be honest with ourselves, the most a lot of us can do is two subjects, and even that sometimes feels like a lot.
I’ve also see someone study for three hours non-stop, and I wondered if they stayed focussed all that time.
If you happen to be a gifted human, who can manage four subjects in a day, and go on for an uninterrupted three hours, congratulations!
But I would not recommend this for us ordinary mortals.
Science says we can barely go beyond one hour without losing focus. The time varies for different people. Know what your own time limit is, and fix your study sessions in this limit.
In case you want to be sly and go too easy on yourself, the minimum is thirty minutes.
Note that, like endurance in sport can be increased, so can the time you are able to stay focussed. If you work at it, you can stretch your attention span.
About how much to do in one session, I suggest you do short chapters in one session. Like five to ten pages or so. If the topic for that day is long and challenging, cut it into divisions according to its subheadings. Do one or two divisions in one session. You are not restricted to one session, so you can have a couple of separate sessions, with breaks in between. This way, you can finish the topic in one day, as planned.
Putting too much on your plate will overwhelm you. Have you ever picked up a novel, and its thickness made you feel lazy to read it? Same thing happens if you plan to go through a large number of pages.
So set small, realistic goals, so that you don’t feel lazy or lose morale because there’s too much to do.