Start planning if you can. Although the situation described above sometimes occurs, it`s also very common for professors to give their students a fairly detailed idea of what an essay question will involve in advance of the test day.
On the topic of planning, it`s important to sketch out an idea of how long you want to spend on each section of your essay. If you know the number of paragraphs you`ll need to write ahead of time, you can do this before the exam even starts!
Consider practicing writing under time pressure. You`ve probably written dozens of essays before–the only thing that sets a timed essay apart is that it`s timed. Students often struggle to complete the full essay within the time constraints, particularly if they have to write longhand when they`re accustomed to working on the computer.
Without a clear plan, you run the risk of realizing partway through that you`ve drifted off topic or written yourself into a corner, and fixing these mistakes will consume a ton of extra time.
If you did your best to write a complete, clear, and insightful essay within the time allotted, you should have nothing to worry about.
For this reason, it can be helpful to simulate the conditions of a timed exam before the actual day: pick a practice question, find some lined paper, set a stopwatch, and see how you do!