I don't remember how much time I took for my essay exam, but I needed more time than I thought to need, because you need to write in English (and that's not my native speech), so sometimes I had to reformulate sentences 3-4 times to make it understandable

And like Roberto already said: the questions are open questions, so the more you can tell about your assignment, the better (if it's on topic or related to the topic of course).
When you developed your assignment on your own, you don't need to worry at all, this part will be the easiest you have done for this assignment
Good luck!