Once you are used to flying transcon flights in Y*, 6 hours from east to west and 5 hours from west to east become super easy. That said, there was a recent flight on AS which flew for 7 hours+ to get into SFO from JFK, and we flew at least an hour under 20,000 feet at a speed around 400 mph.

*combinations of 1) cannot buy F outright due to work policy; 2) no F seats left; 3) no upgrades to be had on hub-to-hub flights