I started packing it at about 30 weeks I think, just adding things bit by bit, then I think it was ready around 35-36weeks.
I still find myself going to it now thinking "did I do that?" and checking things, so its never really done I don't think.
I ended up totally re-doing everything cuz I'd went way OTT with things I'd packed. I decided to take the essentials and if I need anything else OH can come home and get things for me, or the Mother In Law says just to let her know if I need anything and she can grab things.
I'd say don't be too organised too soon, you'll leave yourself with nothing to do, I think I packed mine too soon.