Final answer:
PTAs should wash their hands for at least 20 seconds before and after patient contact, in accordance with CDC guidelines.
Step-by-step explanation:
The minimum amount of time that a Physical Therapy Assistant (PTA) should wash their hands before and after contact with a patient is 20 seconds. This recommendation aligns with the guidelines set by the Centers for Disease Control and Prevention (CDC) to ensure that health-care workers thoroughly clean their hands to prevent the spread of pathogens. These guidelines specify that a proper handwashing should cover all parts of the hands and fingernails, and can involve humming the "Happy Birthday" song from beginning to end twice to keep time.