If you are the person who is documenting a medical power of attorney then you would be called as principal. Any type of medical POA becomes effective only when you are incapacitated unless and otherwise specified by you that it takes effect immediately. Generally health care directives do not expire until your death or unless you revoke. The health care directives are intended to create an agent on your behalf who makes decisions concerning your last stages of life. This document should meet certain requirements to become enforceable by law.
The name of the parties (principal and the agent) should be clearly mentioned in the document. Write full names along with social security number or a tax identity number in case you choose a company as your agent. The agent selected by you should be an individual only and should not be your health care provider or an employee working in the clinic or hospital of your physician, unless he is your relative.
While signing the medical power of attorney you can indicate a date for validity if needed. Otherwise, it will be considered it is valid until your death. However the power of the agent becomes effective only when you become incapacitated or mentally sick. If you wish you can include a specific date while creating a document. Generally the document is valid until the principal dies or unless he revokes the power.
You can mention the powers of the agent separately in attached paper indicating clearly what are your requirements and what circumstances you may require life support devices and more. Many states have made it mandatory to sign the document before notary public. Always make some copies of the medical power of attorney document and keep it in a safe place. You can also give a copy to your doctor, health care center, spouse and children and also important relatives and resource person.