The longest I had in duration was, ironically, a DEN-RAP flight. Normally those block at less than an hour, but this was on the first snowstorm of the year and Denver was particularly derailed. We pushed back from the gate and sat for three hours before even starting an engine. Then they needed the gate so we parked at a pad to wait to get in line for deicing. Deicing was a complete circus. Passengers were all understanding, but I could see how that could have turned bad. When it was all said and done we blocked over 6 hours on that leg. This was obviously before the passenger bill of rights or whatever they have now. I also remember it being the day that United was parking their last 737 because that plane got caught up in the storm too, but got "VIP" status from atc.