This is always an oral examination conducted by the Exam Committee. There is no rule governing the duration of the General Exam, but it is usually in the neighborhood of two hours long. Some Advisors want their students to begin the Exam with a brief expository talk reflecting the advanced reading they have been doing. In any case, the Committee members may ask any mathematical questions they deem appropriate. It is a good idea to talk in advance with each of your Committee Members about the general area of Mathematics about which he or she will probably question you.
Most students feel at least slightly anxious about an oral exam.
Actually, it is an informal process which has many advantages over a written exam. It is alright not to know how to answer some of the questions, and it is also alright to get hints or reminders to get you started on a solution. It is best to try to relax, not be afraid of thinking on your feet nor of making some mistakes. In any case, this is your last general examination in Mathematics. The Final Examination for the PhD is a Dissertation Defense, and is an occasion to present your results to your Advisory Committee for its approval.