You start making more "simple" mistakes - and even falling in the water occasionally too (or more often than usually, depending on your type of riding if it involves lots of "splashes" normally)
But for me, I also have to get "dialed in" too, so the first hour or two on the water is usually "warmup" physically - and from then on I am on a roll and can kite in good waves for 5-7 hours typically, with max body "performance" (power and balance).
Sometimes, or often, after that point, I get tired - and the signs are typically that you miss some moves you did with ease the ealier hours
The good thing is, that as long as you know this is the case, you can continue a couple of hours if conditions are REALLY good, because you still get the same "good feel" from awesome waves - even when you dont hit them as aggressive and perfect
So KNOWING your fatique has set in, is quite important, and when you know, you can sometimes continue riding with good results, OR stop because it will become dangerous, if you know exactly where your "limits" are, at a given fatique level
It comes with time (learning)