The optimal amount of time in a single study session will vary for each student, however it will usually be between 20 and 40 minutes of studying. Most students spend far too long in a study session. You’re much better having more, shorter sessions than one big marathon. One of the most consistent findings from research into learning is that short sessions spaced out over time are more effective than big blocks.
After your study, take a short break to allow the information leave your short term memory and then give yourself a quick test on what you’ve studied. As well as being a useful reality check, getting your mind to recall the information is a very important part of the learning process.