"[Other authors] have challenged the "2-hour rule" on the grounds that a longer time, that is, at least 4 hours, is necessary before concluding that the active phase of labor has failed. We agree."

- Cunningham, et al., from Williams Obstetrics, 23rd ed., in the chapter on Abnormal Labor

These kinds of things are why I like reading textbooks over transcriptions (even at the point when I'm supposed to be cramming everything the night before the exam).