Good suggestions here. I’ll add this: work with your course head on some rubrics. They will help you and your students. A good resource on this is the book by Stevens and Levi. Your local teaching-and-learning center may be able to get you a copy.
What rubric are you using? The rubric will determine the amount of time. If you don’t have a rubric, make one! There is a good book by Stevens and Levi.