I am not aware of any research that supports setting arbitrary time limits for labor duration. How could there be? There is a wide range of "normal" around "average" and too many variables in each individual case to set hard and fast rules. To name a few, how is the laboring woman tolerating labor? How is the baby doing? What physical or psychological factors might be impeding progress? What measures have been tried to promote progress and with what result? At some point, clinical judgment dictates delivering the baby, but it is a subjective decision. The truth of this can be found in the large body of studies reporting much lower cesarean rates and equally good neonatal outcomes in low risk women cared for by midwives, and especially midwives at out-of-hospital births, than for similar women giving birth in hospitals. (The point is not that the caregiver was a midwife but the model of care midwives are more likely to practice.) Likewise, we have numerous studies showing extreme variation in cesarean rates among individual obstetricians also showing no relationship between higher rates and better outcomes. This tells us that patience and care that supports the unfolding of the normal process produces better results than forcing labors to fit into a timetable.
-- Henci