At what point in studying should I give up for the night?

When you feel you have achieved what you set out to do, if you feel you have completed your set task then you can relax :)

I believe it's best to pack up once you feel that you have studied enough to help you reach your goal (whether that is a test or assignment that's coming up). As long as you know that have studied enough for it to benefit you, it should be a suitable time for you. It's also important to make sure that you have enough time for you to sleep at least 8-10 hours before you wake up the next day. It's always best to be well-rested in order for you to retain everything that you studied the night before properly.

As a student with upcoming IGCSE myself, the prospect of not having studied enough or being underprepared can be quite frightening. Although, your studies are important, remember, your health is also extremely important. Without your health, the fruits of your studies cannot be maximised. Personally, I did suggest you keep a appropriate timetable, although this is overused advice, its a practical and reasonable one. The timetable should be reasonably arranged. There should be a time set out for leisure and rest-Sleeping ofcourse, would be better if its at night, so devote more of your work and study shedule for your day, and your play and sleep for the afternoon and night. Also, your objectives for your studies should also be achievable within that timetable. For me, if I set out a lofty goal and was unable to accomplish it, makes it difficult to get a good night sleep. So set objectives are achievable in the time you have set aside. I did recommend sleeping before the day changes, anyway.

Sometimes just because you are "studying", that doesn't mean you are actually retaining information. Put down your book or flash cards or whatever, and ask yourself what you just read or did 15 minutes ago. If you cannot remember what you were doing, then it is time to take a break. Usually after around 30-45 minutes, mental fatigue starts to occur, but if you are tired, then this could come even sooner.