I packed mine on the day they induced me I didn't even know they were doing it that day. So i guess you'll just know in your gut when to pack it lol. And most likely you won't use any of the stuff in there because they have it at the hospital already. Only thing I used in my bag was my own 2 hospital gowns. Oh and make sure you bring recieving blankets because they dont let you keep the ones there. Good luck.
I barley finished mine yesterday. I actually stressed on it until it was done lol yet I kept procrastinating. If you wanted to you could get it ready now :)