That's a popular question from drivers.  Long answer short, it's not an automatic 2 years, and never was.   Prior to the National Registry of Certified Medical Examiners some drivers were probably accustomed to receiving a 2 year card regardless of their medical diagnosis or condition (i.e. diabetes, hypertension, cardiac disease etc.).  Now, most likely than not the certified medical examiner that you are now seeing "Is" applying the FMCSA driver qualification rules and regulations as required by training, and may give you a medical card for less than a period of 2 years due to certain medical conditions that you have. 

Remember, even if you do get a medical examiner's certificate for the full 2 year period, and later return from an illness or injury that interfered with your ability to operate a commercial vehicle, you will have to undergo another medical examination even if the medical examiner’s certificate has not yet expired.  Drivers with medical variances (Federal Vision and Diabetes Exemption Programs) require annual medical certification.

So when does your card expire exactly?

The expiration date listed on your medical examiner’s certificate means that the certificate expires at midnight of the day, month, and year written on the certificate.

The FMCSA states:

There is no grace period on the expiration. The driver must be re-examined and recertified to continue to drive legally.

Randolph Rosarion M.D.