I don't know, really. I went through all four years with ADD-inattentive. Recently diagnosed, it's like a different universe on Adderall. I can read a paper without stopping now. Back when I was actually in school, I'd spend a few hours studying, but 75% of that would be daydreaming about exceedingly arbitrary subjects. I remember one day I wasted an hour daydreaming about being a pirate. I seriously wish I was joking. I made it through fine grades wise - always had a 70% or better in everything - but I was held back due to forgetting to take a test I needed to make up and the professor thought I was usurping his authority. I seriously did just forget about it...but the school would have none of it and they threw the book at me. (I soon found that they would further enjoy doing that several more times....long story....I don't think they like me much, honestly... 🙁 )
That said, you can work as many hours as you can handle....and for some people that is realistically over 30 hours. I don't really have a life so it was rather easy for me to pull off.