Before you start though it's important to note that you should decide before hand, ideally the day before, how long you want to study for including breaks. For example you could set time to study from 3pm-6pm and then stick to it! From the first day that you do this you are going to want to increase your studying time gradually, studying stamina can be increased trust me! I've used this method to get up to 4 hours straight! So the second day instead of your first studying session being 30 minutes increase it to 35 minutes then take a 20 minute break. Then you could study for 40 minutes, take a 20 minute break and if you don't think you can keep increasing from there just do 35 minute study sessions at a time for the rest of your time that you set apart. The 3rd day you would study 40 minutes at first and then 45 minutes and then 40 for the rest of the sessions of that day. You just keep increasing your study time a little everyday with breaks and soon you will be able to study with intense focus for hours without tiring, and then take a break and study for more hours! If you have any questions about this method feel free to ask :)

I'd recommend doing a method that helps you improve the amount your'e able to study. This is what you do; Say you are only able to study for 30 minutes before you start to lose concentration and aren't as focused. So assuming that what you need to do is study for 30 minutes straight where you actually time it and are focusing intently on your studies, and as soon as the timer goes off stop studying, then take a 10-15 minute break where you don't do much of anything. This includes going on your phone or on your computer or ANYTHING! what you do in your breaks is important, which should be next to nothing! Okay so you may ask what can I do in my break? Well you can go for a nice 10 minute walk outside and get some fresh air, you could do some push ups, you could drink some water, you could take a 15 minute nap, meditate, do some yoga,etc. After your relaxing break do another session of studying for 35 minutes and then take a 10-15 minute break again. Then once again you can study again for 30 minutes or 35 depending on how good you feel. You can repeat this process for however long you can stand without burning yourself out.

Who told you your brain doesn't get tired?!?! You should only be studying for 45 minutes at a time, then take a break. Anything after 45 minutes won't be retained as easily and your brain will be more likely to make mistakes later. If you play a musical instrument, you should study for up to 45 minutes, then take a break and play some music. You'll be surprised at how much of a difference it makes. Just a tip :)

when i study for a long time i really feel tired too .. I have exams now but it makes me feel so much better to take a little break to have fun or go out with friends. try to have a break when you lose concentration and just enjoy your time then your brain will feel good and will be able to study even much better. good luck :)