Q. How did the five-second dip become standard?

A. Research has proven that a 5-second dip is equally effective as a 160-second dip in promoting root growth. Longer dips, such as a 320-second dip, actually decreased effectiveness.(1)

Reference:
(1) Dirr, Michael and Heuser, Charles. The Reference Manual Of Woody Plant Propagation. Athens, GA: Varsity Press, 1987.