I started a month or so before but I think it comes down too how committed you can be. Start as soon as you have the willpower too, build up buy start going over everything as it all helps. Obviously note that you'll have to look over stuff again the earlier you begin but that will only help you
For me, I was kind of lazy and only really had the motivation to work when it got too the month before so if you can, start early
All the best