My advice is to read it at the beginning of your study plan, then read it again 1-3 nights before the exam. Reading it twice really helps in my opinion. Between Levels I and II, I’ve read Ethics 4 times, and will read it a 5th time in a few months. It’s one of those topics where the more you read through it the easier it becomes to extract the right answer on the exam, as the correct answer is sometimes subtle.
Also, reading Ethics the night before the exam might help you fall asleep (heh) when nerves and anxiety may be running high.