How about a helicopter question for a change.  

How many hours you work is an issue between you and your employer, the FAA doesn't care and the FAR's don't address it.  The regulations limit the amount of time you can spend on duty when such duty involves Part 135 flight.  They also specify how much time must be afforded for rest prior to Part 135 flight duty.

Some certificate holders consider the flight completed when the patient is delivered to the receiving facility.  In that case your planning would involve sufficient time complete the flight to the receiving facility by end of your 14th hour.  What happens next is a matter of company policy reference hotels/crew swap/transportation.  There are no regulatory requirements other than the need for 10 hr rest period prior to your next duty day if it is going to involve Part 135 flight.

To answer your question, if you don't want to work more than 12 hours a day...you might want to work somewhere else.