The first stage of labour continues until your cervix is fully dilated, about 10cm, allowing the baby to move lower through your pelvis. For women having their 1st baby, labour lasts on average for about 8 hours, most of which is the 1st stage of labour. Labour is unlikely to last for longer than 18 hours. It's likely to take less time if it's not your first baby, because your pelvis and vagina have been stretched before.
Attribution: BUPA