I flew business multiple times, Frankfurt-SF (~14h), and it's a breeze if you can lie down and sleep. Not even boring. Jet lag is way worse than the flight itself.
JFK - HKG is 17h30. Even SFO to HKG is 12h30, and those are two cities on the coast in a middle position. (I'd rather pick Shanghai as a comparison but there are no direct flights at this time due to zero COVID so the info is hard to get.)
The longest scheduled flight right now is probably SIN to JFK at 18h30m. Qantas wants to do a 20h JFK-SYD route.
I flew business multiple times, Frankfurt-SF (~14h), and it's a breeze if you can lie down and sleep. Not even boring. Jet lag is way worse than the flight itself.