Seriously, you really don't need to try, you need to come up with a plan that will be effective and efficient. I know from experience that the most I can properly concentrate on revision (which is a pretty intensive mental activity) is 1 hour. And I would say from my experience I can concentrate slightly more easily than the average 16 year old.
Honestly, I'm not trying to be annoying or anything, but you need to take regular breaks, preferably involving some form of physical activity but at the very least they must distract your mind from revision:
- They will keep your mind fresh. After a couple of hours of studying, you won't be able to think about things properly, and taking a healthy 15-30 minute break will allow you temporarily partially forget what you have learnt. When you come back to re-revise you will therefore have a fair understanding of the material which you know, and will be able to relearn everything else. After several repeats of this you should find that the information sticks much more permanently.
- Sitting down for hours on end with minimal breaks is unhealthy and mentally draining. Revision takes a lot of brainpower to do well.
- Revising for hours at a time will mean your concentration will be shoddy. If your mind is wandering around the place, not only are you vulnerable to procrastination, you will be able to absorb information at a quarter of the rate. This is really no exaggeration!
- Attempting such an ambitious plan will inevitably lead to some form of failure, since you are expecting something from yourself that is not physically possible to sustain. Get a realistic plan that you can keep and stick to it. You will feel like you haven't underachieved.
Even if this means that you just do 30 minutes of revision and then taking 30 minutes break, you will gain so much more. Trust me, I know this from experience, having tried to revise for ridiculous time periods in the past.
Anyway, hopefully some of what I've said has been helpful to someone.
Oh, and someone here said that if an intensive strategy such as you suggested works for you then go for it. Respectfully, I'd like to disagree. People seem to have developed a somewhat convoluted idea of what real concentration actually is, and hence believe they can go on for hours. I have never seen such a person.