I think pain relief plays a part too, so I've included what I had

DS - 6ish hours from first contraction to birth (gel IOL at 32w, gas & air/pethidine)

DD1 - 19ish hours from first contraction to birth (gel IOL at 38w, augmented with synto ~10 hours in, gas & air/epidural)

DD2 - Weak irregular contractions for roughly nine hours, then 4ish hours between start of "real" contractions and birth (natural spontaneous labour and birth at 40w)