Your concentration span is the time you can concentrate / pay full attention to a specific subject without being distracted. This varies according to the subject you are studying, your mood, how much sleep you have had, your diet etc.
DO NOT study when your concentration starts to fade; you are not taking in the information you are reading and it is wasted time. Take regular breaks to refresh yourself; it could be a healthy snack, some chilled conversation with friends or family or a brisk walk.
30 minute sessions with a 10 minute break is a good starting formula.