Still a little confused. You mean how long I would travel in a seat before I would upgrade to a cabin, irregardless of the mode, or how long I would travel in a plane seat versus a train seat?

In the US, I can't imagine ever taking the train outside of Amtrak's Acela between Boston and NY. And there I would say I would prefer the train due to comfort, but if I am going only to NYC will choose the plane either because it is half the price, OR the hassles of getting to the train station outweigh getting to the airport.