I currently fly from SAN->DEL, which includes (in addition to 6 hour flight between SAN and EWR) a 16 hour leg between EWR and DEL due to not being able to fly over certain countries. It is a crushing flight, both mentally and physically, even in a lie flat bed and with regular stretching. I can't imagine doing a 24 hour flight in a normal economy seat, and I imagine most people would take a layover over that. 

Ew, no thanks.

Recently I flew from Cape Town to Atlanta, and I think it was 15 and a half hours, which was too much. (The flight from Atlanta was only about 14 I think, but you have a little meal, watch a film, fall asleep, and are you there yet? No, no you are not, you are only halfway there.) A few years ago I did Hong Kong to NYC and I think it was just under 14 hours, that was the longest flight I'd taken at the time (I travel a few times a year for work), but it was mentally rough, not being able to move too much (yes you can walk around but it's not like walking outside or in your house). I am not a fan of over 6 hours, my new favorite long trip is Boston to Dublin -- you're in Europe but it's so close!

Your scientists were so preoccupied.

I travel with Qantas each year from Australia to NYC. The last three trips I've started in Sydney, plane refuels in Auckland NZ and then non-stop to NYC. Avoiding customs and immigration at LAX/SFO is a huge benefit doing the flight this way. The NZ to NYC leg is a bit over 17 hours - the ability to tolerate this depends on whether you are up the front or down the back of the plane. I'm not sure how fun it would be doing the entire trip in economy between Sydney and NYC. At least with my current route there is the opportunity to stretch your legs for a few hours in NZ.

A year-and-a-bit ago my beloved and I flew from the UK to New Zealand, her homeland. It was my first time travelling there and the longest leg, 13 hours and 40 minutes from London to Singapore, was maybe seven hours longer than any flight I had been on before. And then it was eight more long hours to Sydney. And then another three to Christchurch!

I am tall and not economy sized. It was tough.

But then, my wife's grandparents (all of them) left post WW II Europe for New Zealand via ship. It would have taken them about six weeks!

You could get a full nights sleep, and watch 6 or 7 regular length movies, and have time left over for meals, bathroom breaks, and stretching. 

Based on my own experience with 12 hour + flights, everyone getting off this plane is going to be SO WEIRD. They need to have very, very clear instructions for what to do and where to go next. 

Not the longest duration non-stop commercial flight. That also belongs to QANTAS. The double sunrise flights in the 1940s from Perth, Australia to Sri Lanka took 27-30 hours. Flying in Catalina flying boats carrying 3 passengers and mail. Not a lot of room in Catalina, beautiful aircraft though they are.