The Centers for Disease Control and Prevention says to wash your hands for 20 seconds. The World Health Organization says hand-washing should take 40 to 60 seconds. And it's complicated to determine who is right.
The optimal length of time for hand-washing is likely to depend on many factors, including the type and amount of dirt on the hands and the setting of the person washing hands (temperature of water, washing technique, the type of soap, etc.)

There are just too many things living on our hands to wash all of them off.
Killing all the microbes on your hands has never even been the point of hand-washing. The point is to get as big a reduction in microbes as possible while balancing that effort with the demands of real life.