When studing or doing work how long can you go straight before needing a break?!

When studing or doing work how long can you go straight before needing a break?

I've been working on this long assignment,and I find that after a couple hours I lose my train of thought, can't concentrate, and my mind wanders else-where. Does this happen to anyone else? How long can you work at something straight before you should take a break; and, how long of a break do you take?

Well, never work more than 3 straight hours, it'll turn out pretty bad. If you work for 3 hours you can take a 1 hour break before continuing, and if you work less than that, then your break should be shorter so you won't lose time or ideas.
Don't drink coffee or tea, they actually overload your brain and make it even harder to concentrate on your work than thinking of having a break.
Don't work heavily more than total of 9 hours a day (excluding break time)..
Always keep yourself organized, keep everything you need in your reach and don't leave your papers all over your place cause watching a mess makes your brain go messy as well.
If you have more than one day to finish your work, make a simple schedule for your work that includes tasks, date, starting time, and fininshing time. This way you can keep track of your work and be able to catch up if you become delayed for any reason.
Good luck :)