As soon as you realize you're becoming inefficient in your studying, hard stop and do something fun or relaxing, just take a break. The only time I truly sit down and study is about a week before exams (that's also because I attend all lectures in person), but during that week I probably study 12-16 hour days. Does that mean I'm sitting there and 100 percent grinding the entire time? No, that sounds impossible, at least for me. I probably have about 8 hours of intense, 100 percent efficiency studying, and 4 hours of time for myself in little 20 to 30 minute breaks, whether its cooking, cleaning, video games, YouTube, etc. Try to set realistic goals between breaks too (I'll study 2 more lectures, then take a 30 min break, etc). I've also had full days where someone couldn't pay me to study, maybe that's burnout?? I felt no problem just taking the day off. I usually let the stress of the exam gauge my study level haha; if I wasnt pulling out my hair thinking I NEEDED to study that day, I probably dont.