More about drawing… When beginners are learning to draw, they get tired. We all do when concentrating on a difficult task. Like building up your muscles when weight training, the ideal time to keep at it is just a little bit longer than you would like to. I recommend using a timer. If you find that you can work comfortably for 15 minutes, try setting the timer for 20 minutes for the next few days. Continue this until you can comfortably work for about 35 minutes. That seems to be a good maximum goal for intense concentration. It really is like exercising. You are training your mind to concentrate, which can benefit other areas in life as well. All good!