Study for a maximum of 25 minutes and then take a 5-minute break. The ideal study time (and break time) does not exist: it depends on the person and what is being learned. However, it is often the case that we can only concentrate on something for a maximum of 25 minutes. Your brain needs time to process everything and can only handle a limited amount of new information (see cognitive load theory). A short break helps to get back to work refreshed. Studying for hours on end is not very effective. One way to do this is the Pomodoro technique.
Ariga, A., & Lleras, A. (2011). Brief and rare mental “breaks” keep you focused: Deactivation and reactivation of task goals preempt vigilance decrements. Cognition, 118(3), 1873-7383. doi:10.1016/j.cognition.2010.12.007