FWIW one might be able to find their own optimum by carefully listening to their body. In college when I overdid math I had a very distinctive buzz in my head (like the brain "shorting out"), that signified I am done for the day.

Remarkably, the same material that stupefied me just prior to "shorting out" would become crystal clear the next morning. I think the brain needs to be pushed up to a certain limit before it considers the material important enough to be worth of serious "structural changes". The pay-off is non-linear.

The difficult part is that you might be tempted to quit because you're "tired", even before you actually hit the wall, but then blowing past the limit in a fit of zealotry just destroys motivation without any extra benefit. The balance is hard to find. The best I came up with is try to observe my performance diminishing and use that as an "objective" signal. Easier in some areas than others.