With new planes models, long-haul flights would become more common. Emirates’ non-stop flights to Houston and Panama City take about 17+ hours. Qatar Airways started to fly to Auckland, New Zealand, which is also about 17+ hours.
But I would think that the longest flight will be operated by Singapore Airlines in 2018 to New York City which would take 18+ hours.
Non-stop flights are very convenient, especially for business travellers. I had a chance to experience one of those flights in 2014 from Dubai to Houston.
I must say that if you are lucky enough to end up in Business Class, you going to be just fine upon arrival. I am not quite sure that I would say the same about passengers in Economy Class.
Although, planes are equipped with more leg room in Economy (I would think), the flight duration is just too long. But then again, the level of comfort (or discomfort) is still better than few decades ago 🙂