A person should spend enough time hand washing to be able to hum the "Happy
Birthday" song twice--roughly 20 seconds.

A person should wash his or her hands for approximately 20 seconds,
according to guidelines from the Centers for Disease Control and Prevention
(CDC). In order to roughly estimate the recommended amount of time, the CDC
advises humming the “Happy Birthday” song two times while scrubbing the
hands and fingernails before rinsing. Only approximately 5% of people wash
their hands for the recommended time, according to a 2013 Michigan State
University study of hand washing habits in public restrooms. The average
length of time people spend washing their hands falls short of the CDC’s
recommended 20 seconds, at approximately six seconds.