For me, it's pretty low. About 3 hours max if I'm not on a widebody with two seats to the outside rows. If I'm with one other person, I can stand the 2 pair seating for much, much longer since it feels more private and you can put up an armrest, get up any time needed, share space and just feel more comfortable . I did so much economy flying for work where I felt trapped on a row with strangers for 5 or 10 hours and just can't do it on my own dime anymore.

On international flights, I always price out both economy and J or F and see what the difference is and go from there. If the trip is short, I'll be more apt to want to get good rest on the flight. And if the flight it really long, I'll look a lot more for an upgrade. Like flying to Australia. I wouldn't do it in the back. Just no. Europe, I've done plenty of each and have been fine most of the time. Economy an being expected at work the next day is a no-no though. I'm not one of those lucky people who handles jet lag well.