<p>@CHD2013 I think that most people can handle more than 20 minutes of focus at a time. But quality and retention tend to decrease for most people after 20 minutes (even if it’s not noticeable). Let’s take myself as an example. I always excelled in school with very little effort, I have a PhD in mathematics, and I teach both undergraduate and graduate level mathematics. Yet, when I write out solutions to simple problems for more than 20 minutes at a time the quality of my explanations decreases. When I review what I have written later, the content that I had written for the first 20 minutes requires very little editing, while the later content sometimes has to be rewritten completely. </p>
<p>I am not saying that someone who studies 2 hours per day will not do well. All I am saying is that the first 20 minutes is the most productive. If you want to put in the minimum effort with the maximum result, about 20 minutes at a time is the best way for most people.</p>
<p>And let me also emphasize that this is just a guideline. If a student wants to continue practicing for more than 20 minutes I’m not saying that they absolutely shouldn’t. I’m just saying that they don’t have to.</p>