hi there arthur!
I remember when my mom gave birth to my younger brother (she was 39 then), contraction started at 4pm then she gave birth at around 3am..
When I gave birth to my little Therion, the contractions started at 10 am.. gave birth (natural) to my son at exactly 12 midnight.
oh, and we started counting when the contractions are 5 minutes apart...
so, probably 12 hours on average?
