From the US DOT “FlyRights” page (which is woefully inadequate):
Some flights are delayed on the airport “tarmac” before taking off or after landing. DOT rules prohibit most U.S. airlines from allowing a domestic flight to remain on the tarmac for more than three hours unless:
the pilot determines that there is a safety or security reason why the aircraft cannot taxi to the gate and deplane its passengers, or
Air traffic control advises the pilot that taxiing to the gate (or to another location where passengers can be deplaned) would significantly disrupt airport operations.
U.S. airlines operating international flights to or from most U.S.airports must each establish and comply with their own limit on the length of tarmac delays on those flights. On both domestic and international flights, U.S. airlines must provide passengers with food and water no later than two hours after the tarmac delay begins. While the aircraft remains on the tarmac lavatories must remain operable and medical attention must be available if needed.