Our max duty day is 13 hours, or 14 hours if you are staying in that time zone, (or 10 hours, 12 hours, 16 hours, or 18 hours ... depending on the start time, crew compliment, route or aircraft on-board facilities).
If the occurrence you mention is the crew's first mission of the day, then one has a lot of leeway to wait out delays. However, if it is the last leg of a 12 hour day, then maybe not so much time ... even for a short leg.
Usually these duty day limitations, while checked by the crew, are monitored and enforced by Crew Scheduling in consort with Systems Operational Control. Namely for reasons we can not see in the aircraft, the flights may be canceled due to delays. So we would just get a data-link message telling us the flight is canceled long before it becomes a limitation.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!