Original Message

The duty period has not maximum.  Just that if you are going to operate an aircraft under Part 135, you must be able to show 10 hrs of rest (requred rest) before you plan to complete the flight time of that assignment (i.e., land by that time).  Your duty period can go far past that requirement, as long as you are not planning to fly.

We tend to think of it as a 14 hour maximum in practicality, but it isn't!   There is no requirement to end your actual duty period by the 14th hour.  

The FAA has consistently interpreted duty to mean "actual work for a certificate holder, or the present responsibility for work should the occasion arise."   Meaning, it is not rest!   Your rest starts once you no longer are on duty!   That's when you can start your rest timer for the required rest of 10 hours!