Longest flight I've been on was EWR to NRT, about 14 hours. Was lucky, lots of empty seats in Business class, so didn't feel claustrophobic, and back then, I still smoked, and smoking was still allowed on international flights.
Today, 17 hours on a cramped flight, I'm sure I'd have to get some Rx from my doctor, and if I were stuck in coach, forget it. Just wont go. Not because I'm a snob about sitting in coach, but it just gets to me sitting there for more than few hours. When I fly coach, an aisle seat or look for another flight.