I think every student should be taken on a 2-hour lesson until their brain fails, just once.
A good point (though not necessarily practical).
Far, far better that you find out about overload when you're with an instructor rather than when you're out there alone.
I have a rule. When a student starts to push the controls the wrong way after previously getting it right, or simply can't hear what I say when I've said the same thing about three times, or hears but can't send the messages to his/her hands and feet....
It's time to stop. Back we go for a coffee, even if we come out again later. And often a little chat about stress and fatigue. And if it's happened after less than three-quarters of an hour, a chat about individual differences, recognising one's own limitations, and not getting competitive or trying to be superman/woman.
You see, you can learn an awful lot without actually doing any flying...or paying for it.
Michael, I bet your instructor learned something that day too.