As a former airline pilot, I'm not aware of any rules limiting how long you can be kept on aircraft. I don't think it's covered by the Geneva Convention, although it probably should be. I do know a few things: Once you are off 'the gate', the crew is being paid. Once off the 'gate', the flight is considered on its way, and the airlines hate to have flights come back - it screws up their nice little statistics (on time arrivals, completed flights, etc.). The airlines work great, when the variables are severly limited. But throw in a little weather, etc. and the whole system can really come crashing down. Six hours is inhumane though. The crew was probably sympathetic to your situation. Airline management? well, they don't care. Period.
That's the thing, spower. The crew was TOTALLY unsympathetic. As we approached hour number 4, I asked the "flying waitress" (she didn't deserve the title flight attendant) how long they were going to keep us locked up in the plane. Her answer: "have you been drinking?" (!!!) I wasn't rude, didn't raise my voice, used no foul language, just asked her how long they were going to make us sit there. Then she brought the thirteen-year-old "pilot" back to ask if I'd been drinking. It was ludricrous. One of those "how long has it been since you've beaten your wife" kind of questions. THE CREW were making a scene, NOT ME!!