The NHS suggests having your bag packed about three weeks before your due date, because babies are not famous for reading calendars. If you're planning a home birth, it's still worth packing one — a just-in-case bag that stays in the cupboard is a lot nicer than a 3am scramble.
You need less than the shops would have you believe. Hospitals provide the medical things; you're mostly packing for comfort, and a birth partner can always be dispatched for anything forgotten. Borrowed or second-hand is perfect for almost all of this.
One honest tip from parents who've been there: pack it as two bags (labour, and after), and put the going-home outfit for the baby somewhere your partner can actually find it.