Keep records of your estimates and the actual study times. The ability to concentrate varies with the individual. Some persons seem to have phenomenal natural gifts to concentrate on specific tasks. Most of the rest of us are forced to cultivate that skill. If you enjoy a football game on television, you find it easy to devote your attention to the game.
Time even seems to pass faster.
If you think that a particular chapter in your reading will be boring, you are sabotaging your ability to concentrate. So try to find, somewhere within you, an interest in what you are reading. The material you are reading may, in fact, be boring. You must, however, still master the material. So, choose a quiet place, set a time limit, and approach your assignment as something that must be done.