This seems true for any sport. Three plus hours of hard exertion is close to pro level in most endeavors. 3.5 hours of hard Mountain biking is the max for my personal enjoyment, and it's usually a sufferfest starting at 3.0 hours. I'll go after 5-6 all day epics throughout the year, and I love those days of limit testing on the rivet, but they are the exception and not the rule.
Tennis is the same, 3 hours of tennis and my knees cry and my strokes get fugly.
Skiing is pretty similar, though, I can stretch the day as long as needed with some pacing and cruising. And, I admit, I like cruising to break up the day, and that is one of the gifts of a gravity sport, there are moments of just easy balancing and not always needing extortion for forward movement. Also, o bigger hills with varied terrain and long vertical I tend to ski an area/ feature, stop and ruminate with my buds about it and then seek out the next bit of interesting pitch and do the same. At 2000ft or more I don't do too many Summit to lift burners after the first hour or so.
Yeah, usually after hour one, I am skiing at my best, and after hour three my turns get a little 'ragged.'
Then again, on most days, this pattern follows the patterns of snow condition. Except on Spring days, The best conditions, in terms of ease of skiing, are at opening and as the day wears on so does the snow. Powder days being the most obvious example of this reality, but it's the same with groomers, etc on most days. So there's the double whammy of increased snow difficulty and fatigue as the ski day wears on. Knowing when enough is enough varies person to person, but everyone succumbs to these twin realities eventually.
Neat Topic, JB: Effects of time on technique and what that means to your ski day.