I experienced an approximate 8 hour tarmac delay out of YYZ several years ago. We were headed from YYZ to BGI in later November when a major winter storm hit.

Our Departure was scheduled for around 7am, but that was pushed back to 9am the evening before. I knew when we boarded that there was going to be some major ground delays due to heavy snowfall and de-icing requirements. We pushed back about 90 minutes late because of our slot for the de-icing bay, and the wait at de-ice was much longer than expected, probably 60 minutes before we got in. After the de-ice was completed around noon we waited almost 45 mins for our departure clearance which exceeded the hold over time and required a 2nd de-ice.

Unfortunately at that point the a/c engines had been running for about 2 hours and we then required more fuel. Presumably the pilots we also accounting for another 60+ minutes for a 2nd de-ice and departure. So we taxied back and waited for a fuel truck for another 45 minutes.

It was past 1pm at this point and my spidey sense told me were in for more issues...as I had predicted to my cell seat mate our crew announced that a couple of the flight attendants were no longer legal to complete the flight as they would be exceeding their duty times. So we'd have to return to a gate and wait for replacements. It took longer to find a gate that to find a crew, by about 3pm we had a gate and a crew was waiting to board. This was the opportunity to get the additional fuel and apparently the flight also required a lav service and more water.

4pm we pushed back for the 2nd time, finished our de-ice by about 5:15pm and were air born by 530pm.
The crew was very good about providing updates and beverages throughout the day. They were just as frustrated as the passengers. Dinner (aka sandwich) was served to everyone and an alcoholic beverage both 'on the house' as soon as we cleared 10,000'.

We lost a full day of our trip, but it was winter time in Toronto and well...weather happens.