The short is answer is that starting early is better.
Law school is time consuming. If you wait until a couple of weeks before finals to start outlining, you are not giving yourself adequate time. Generally, you should start outlining after the first few weeks of class; and at the latest, by October. If you are reading this guide during the last week of November and have not started outlining, don't panic! Though starting early is definitely better, you can still benefit from outlining. Remember, it is always better to outline, no matter the start date, then to not outline at all.