Concentration is the ability to focus on one task at a time and tune out everything else. However, maintaining this state for more than two hours at a time is grossly unrealistic.
The ability to concentrate is also subject to fluctuations because neurons need time to adapt to different stimuli. Instead of trying to be focused all the time, it is better to learn to use the highs and lows of one’s ability to concentrate.
It’s similar to sports, where you always need to alternate between exertion and relaxation. In the same way, the brain needs relaxation phases in between to regenerate and process what it has learned. “Therefore, interval learning, where you alternate between high and low concentration, is actually best, where you can do light work during a low phase.