You're asking the wrong questions about studying
People get stuck on the wrong metric. They ask "how long should I study for?" when the real question is how you use the time. You can get more done in a focused 30-minute sprint than in a three-hour haze of social media and rereading the same paragraph.
It isn't about the clock. It's about your attention.
So, how do you fix your attention? Treat it like a muscle. You don’t walk into a gym and lift the heaviest weight on day one. Start small. A 25-minute focus session, then a 5-minute break. It’s called the Pomodoro Technique, and it works because it respects that your brain has limits. String a few of those together and you’ve done a solid block of work without burning out. An app like Trider can run the timers for you and build a streak to keep you going.
Your brain is built to forget
Your brain isn't a hard drive. You can't just download information and expect it to stick. Forgetting is the default setting. To fight it, you have to be active.
Reading your notes over and over is a waste of time. Instead, force your brain to pull the information out. That's active recall. Turn your notes into questions and quiz yourself. Use flashcards. Better yet, try to explain the concept to someone else. If you can't explain it simply, you don't really get it.
And you should space out your reviews. Instead of cramming, look at the material the next day, then in three days, then in a week. This tells your brain the information is important and worth keeping.
I remember staring at a textbook for an intro psych class, the words blurring together. I glanced at the clock on my 2011 Honda Civic's dashboard: 4:17 PM. I had been "studying" for two hours and couldn't recall a single thing from the last chapter. That was the day I stopped reading and started quizzing.