What is the maximum number of effective study hours a language learner can do in a day?

I've been studying Mandarin as a hobby for 2 hours a day. Currently I'm at A2 level.

2 hours doesn't sound like a lot, but I already feel quite drained after those two hours of intensive reading/listening/writing/speaking practice. I start to blank out and unable to make meaning of the words I read and the sounds I hear and have to stop.

Even if I quit my job to study full-time, I don't think I can realistically get more than 2 or 3 hours of effective study in. Then after 5-6 days of continuous study, I have to take a day off to prevent burnout.

What do you think is the highest ceiling for the number of effective study hours a learner can get in a day? How do students of intensive full-time language programs manage it without burnout and fatigue?