1st 9 hours (induced with syncto drip), 2nd 6 hours (all natural), 3rd was 3 hours (all natural). For me, it definitely got quicker. With DD3, for the first 2hrs, 55 min I only got to 5cm, then when my waters broke I dilated really quickly and she was born in 2 minutes.
I would say be prepared for a quickish labour. Oh, and all of mine have come between 37-40 weeks.